diff --git a/ChangeLog-8.0.md b/ChangeLog-8.0.md
index c149cc8211f..2e8230370cf 100644
--- a/ChangeLog-8.0.md
+++ b/ChangeLog-8.0.md
@@ -7,6 +7,7 @@ All notable changes of the PHPUnit 8.0 release series are documented in this fil
 ### Changed
 
 * Implemented [#3133](https://github.com/sebastianbergmann/phpunit/issues/3133): Enable dependency resolution by default
+* Implemented [#3236](https://github.com/sebastianbergmann/phpunit/issues/3236): Define which parts of PHPUnit are covered by the backward compatibility promise
 * Implemented [#3244](https://github.com/sebastianbergmann/phpunit/issues/3244): Enable result cache by default
 * Implemented [#3288](https://github.com/sebastianbergmann/phpunit/issues/3288): The `void_return` fixer of php-cs-fixer is now in effect
 * Implemented [#3332](https://github.com/sebastianbergmann/phpunit/issues/3332): Deprecate annotation(s) for expecting exceptions
diff --git a/src/Exception.php b/src/Exception.php
index d5d34417ec1..833b49a8763 100644
--- a/src/Exception.php
+++ b/src/Exception.php
@@ -10,7 +10,7 @@
 namespace PHPUnit;
 
 /**
- * Marker interface for PHPUnit exceptions.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Exception
 {
diff --git a/src/Framework/AssertionFailedError.php b/src/Framework/AssertionFailedError.php
index 1dcff604a7f..0ba25286f6c 100644
--- a/src/Framework/AssertionFailedError.php
+++ b/src/Framework/AssertionFailedError.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * Thrown when an assertion failed.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class AssertionFailedError extends Exception implements SelfDescribing
 {
diff --git a/src/Framework/CodeCoverageException.php b/src/Framework/CodeCoverageException.php
index 084e6bcbf93..36b07231301 100644
--- a/src/Framework/CodeCoverageException.php
+++ b/src/Framework/CodeCoverageException.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 class CodeCoverageException extends Exception
 {
 }
diff --git a/src/Framework/CoveredCodeNotExecutedException.php b/src/Framework/CoveredCodeNotExecutedException.php
index d01f4db44f7..78f89bc39ee 100644
--- a/src/Framework/CoveredCodeNotExecutedException.php
+++ b/src/Framework/CoveredCodeNotExecutedException.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class CoveredCodeNotExecutedException extends RiskyTestError
 {
 }
diff --git a/src/Framework/DataProviderTestSuite.php b/src/Framework/DataProviderTestSuite.php
index 5f04cfbceff..0a465e680f9 100644
--- a/src/Framework/DataProviderTestSuite.php
+++ b/src/Framework/DataProviderTestSuite.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class DataProviderTestSuite extends TestSuite
 {
     /**
diff --git a/src/Framework/Error/Deprecated.php b/src/Framework/Error/Deprecated.php
index 5db12061842..566865f1fe1 100644
--- a/src/Framework/Error/Deprecated.php
+++ b/src/Framework/Error/Deprecated.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework\Error;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Deprecated extends Error
 {
     public static $enabled = true;
diff --git a/src/Framework/Error/Error.php b/src/Framework/Error/Error.php
index 608a065071e..895f92248fe 100644
--- a/src/Framework/Error/Error.php
+++ b/src/Framework/Error/Error.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\Exception;
 
 /**
- * Wrapper for PHP errors.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class Error extends Exception
 {
diff --git a/src/Framework/Error/Notice.php b/src/Framework/Error/Notice.php
index 91c3b8691d2..309af987d0b 100644
--- a/src/Framework/Error/Notice.php
+++ b/src/Framework/Error/Notice.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework\Error;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Notice extends Error
 {
     public static $enabled = true;
diff --git a/src/Framework/Error/Warning.php b/src/Framework/Error/Warning.php
index c364a1389a1..b53493dfadf 100644
--- a/src/Framework/Error/Warning.php
+++ b/src/Framework/Error/Warning.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework\Error;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Warning extends Error
 {
     public static $enabled = true;
diff --git a/src/Framework/Exception.php b/src/Framework/Exception.php
index d28041a90d7..d7c50e73a08 100644
--- a/src/Framework/Exception.php
+++ b/src/Framework/Exception.php
@@ -30,6 +30,7 @@
  * the parent would break the intended encapsulation of process isolation.
  *
  * @see http://fabien.potencier.org/article/9/php-serialization-stack-traces-and-exceptions
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class Exception extends \RuntimeException implements \PHPUnit\Exception
 {
diff --git a/src/Framework/ExceptionWrapper.php b/src/Framework/ExceptionWrapper.php
index 0c1ea5f1be7..913bcb5b319 100644
--- a/src/Framework/ExceptionWrapper.php
+++ b/src/Framework/ExceptionWrapper.php
@@ -20,6 +20,8 @@
  *
  * Unlike PHPUnit\Framework_\Exception, the complete stack of previous Exceptions
  * is processed.
+ *
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ExceptionWrapper extends Exception
 {
diff --git a/src/Framework/ExpectationFailedException.php b/src/Framework/ExpectationFailedException.php
index 6246db7aded..f7d7a9c7453 100644
--- a/src/Framework/ExpectationFailedException.php
+++ b/src/Framework/ExpectationFailedException.php
@@ -17,6 +17,8 @@
  * The exception contains the error message and optionally a
  * SebastianBergmann\Comparator\ComparisonFailure which is used to
  * generate diff output of the failed expectations.
+ *
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ExpectationFailedException extends AssertionFailedError
 {
diff --git a/src/Framework/IncompleteTest.php b/src/Framework/IncompleteTest.php
index 502fe828d3e..268957c03d7 100644
--- a/src/Framework/IncompleteTest.php
+++ b/src/Framework/IncompleteTest.php
@@ -10,8 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * A marker interface for marking any exception/error as result of an unit
- * test as incomplete implementation or currently not implemented.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface IncompleteTest
 {
diff --git a/src/Framework/IncompleteTestCase.php b/src/Framework/IncompleteTestCase.php
index 6b51e4d704f..6cb3066d032 100644
--- a/src/Framework/IncompleteTestCase.php
+++ b/src/Framework/IncompleteTestCase.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * An incomplete test case
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class IncompleteTestCase extends TestCase
 {
diff --git a/src/Framework/IncompleteTestError.php b/src/Framework/IncompleteTestError.php
index a53814daa40..65f9c8bc343 100644
--- a/src/Framework/IncompleteTestError.php
+++ b/src/Framework/IncompleteTestError.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class IncompleteTestError extends AssertionFailedError implements IncompleteTest
 {
 }
diff --git a/src/Framework/InvalidCoversTargetException.php b/src/Framework/InvalidCoversTargetException.php
index 865f7cc354c..ebf2994a94e 100644
--- a/src/Framework/InvalidCoversTargetException.php
+++ b/src/Framework/InvalidCoversTargetException.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class InvalidCoversTargetException extends CodeCoverageException
 {
 }
diff --git a/src/Framework/InvalidDataProviderException.php b/src/Framework/InvalidDataProviderException.php
index 0827d7e1fd5..7e2ef24c637 100644
--- a/src/Framework/InvalidDataProviderException.php
+++ b/src/Framework/InvalidDataProviderException.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class InvalidDataProviderException extends Exception
 {
 }
diff --git a/src/Framework/MissingCoversAnnotationException.php b/src/Framework/MissingCoversAnnotationException.php
index 0b2a5763916..567a6c4c581 100644
--- a/src/Framework/MissingCoversAnnotationException.php
+++ b/src/Framework/MissingCoversAnnotationException.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class MissingCoversAnnotationException extends RiskyTestError
 {
 }
diff --git a/src/Framework/MockObject/Builder/Identity.php b/src/Framework/MockObject/Builder/Identity.php
index 4958ee21ce1..a68bfadf9dc 100644
--- a/src/Framework/MockObject/Builder/Identity.php
+++ b/src/Framework/MockObject/Builder/Identity.php
@@ -10,12 +10,7 @@
 namespace PHPUnit\Framework\MockObject\Builder;
 
 /**
- * Builder interface for unique identifiers.
- *
- * Defines the interface for recording unique identifiers. The identifiers
- * can be used to define the invocation order of expectations. The expectation
- * is recorded using id() and then defined in order using
- * PHPUnit\Framework\MockObject\Builder\Match::after().
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Identity
 {
diff --git a/src/Framework/MockObject/Builder/InvocationMocker.php b/src/Framework/MockObject/Builder/InvocationMocker.php
index e88ea0507f5..bca4a5a983d 100644
--- a/src/Framework/MockObject/Builder/InvocationMocker.php
+++ b/src/Framework/MockObject/Builder/InvocationMocker.php
@@ -17,12 +17,7 @@
 use PHPUnit\Framework\MockObject\Stub\MatcherCollection;
 
 /**
- * Builder for mocked or stubbed invocations.
- *
- * Provides methods for building expectations without having to resort to
- * instantiating the various matchers manually. These methods also form a
- * more natural way of reading the expectation. This class should be together
- * with the test case PHPUnit\Framework\MockObject\TestCase.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class InvocationMocker implements MethodNameMatch
 {
diff --git a/src/Framework/MockObject/Builder/Match.php b/src/Framework/MockObject/Builder/Match.php
index 91e7916fcc0..d343eacfb80 100644
--- a/src/Framework/MockObject/Builder/Match.php
+++ b/src/Framework/MockObject/Builder/Match.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework\MockObject\Builder;
 
 /**
- * Builder interface for invocation order matches.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Match extends Stub
 {
diff --git a/src/Framework/MockObject/Builder/MethodNameMatch.php b/src/Framework/MockObject/Builder/MethodNameMatch.php
index dcd0e31ef45..f4b1150b5ad 100644
--- a/src/Framework/MockObject/Builder/MethodNameMatch.php
+++ b/src/Framework/MockObject/Builder/MethodNameMatch.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework\MockObject\Builder;
 
 /**
- * Builder interface for matcher of method names.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface MethodNameMatch extends ParametersMatch
 {
diff --git a/src/Framework/MockObject/Builder/NamespaceMatch.php b/src/Framework/MockObject/Builder/NamespaceMatch.php
index 0d406a8f059..e4651cce323 100644
--- a/src/Framework/MockObject/Builder/NamespaceMatch.php
+++ b/src/Framework/MockObject/Builder/NamespaceMatch.php
@@ -10,9 +10,7 @@
 namespace PHPUnit\Framework\MockObject\Builder;
 
 /**
- * Interface for builders which can register builders with a given identification.
- *
- * This interface relates to Identity.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface NamespaceMatch
 {
diff --git a/src/Framework/MockObject/Builder/ParametersMatch.php b/src/Framework/MockObject/Builder/ParametersMatch.php
index 8919f965eb0..42bd14679b7 100644
--- a/src/Framework/MockObject/Builder/ParametersMatch.php
+++ b/src/Framework/MockObject/Builder/ParametersMatch.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\MockObject\Matcher\AnyParameters;
 
 /**
- * Builder interface for parameter matchers.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface ParametersMatch extends Match
 {
diff --git a/src/Framework/MockObject/Builder/Stub.php b/src/Framework/MockObject/Builder/Stub.php
index 53ff3fea51b..f4f3d4077a3 100644
--- a/src/Framework/MockObject/Builder/Stub.php
+++ b/src/Framework/MockObject/Builder/Stub.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\MockObject\Stub as BaseStub;
 
 /**
- * Builder interface for stubs which are actions replacing an invocation.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Stub extends Identity
 {
diff --git a/src/Framework/MockObject/Exception/BadMethodCallException.php b/src/Framework/MockObject/Exception/BadMethodCallException.php
index 5178b7b2577..7e655e235a9 100644
--- a/src/Framework/MockObject/Exception/BadMethodCallException.php
+++ b/src/Framework/MockObject/Exception/BadMethodCallException.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework\MockObject;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class BadMethodCallException extends \BadMethodCallException implements Exception
 {
 }
diff --git a/src/Framework/MockObject/Exception/Exception.php b/src/Framework/MockObject/Exception/Exception.php
index e30bbf32e54..53cbed2aaa1 100644
--- a/src/Framework/MockObject/Exception/Exception.php
+++ b/src/Framework/MockObject/Exception/Exception.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework\MockObject;
 
 /**
- * Interface for exceptions used by PHPUnit_MockObject.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Exception
 {
diff --git a/src/Framework/MockObject/Exception/RuntimeException.php b/src/Framework/MockObject/Exception/RuntimeException.php
index cac3a40b4d5..33b6a5be32f 100644
--- a/src/Framework/MockObject/Exception/RuntimeException.php
+++ b/src/Framework/MockObject/Exception/RuntimeException.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework\MockObject;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class RuntimeException extends \RuntimeException implements Exception
 {
 }
diff --git a/src/Framework/MockObject/Generator.php b/src/Framework/MockObject/Generator.php
index 22171c03200..4e1a1052f90 100644
--- a/src/Framework/MockObject/Generator.php
+++ b/src/Framework/MockObject/Generator.php
@@ -22,7 +22,7 @@
 use Traversable;
 
 /**
- * Mock Object Code Generator
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Generator
 {
diff --git a/src/Framework/MockObject/Invocation/Invocation.php b/src/Framework/MockObject/Invocation/Invocation.php
index d0367ac07e6..27556926925 100644
--- a/src/Framework/MockObject/Invocation/Invocation.php
+++ b/src/Framework/MockObject/Invocation/Invocation.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework\MockObject;
 
 /**
- * Interface for invocations.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Invocation
 {
diff --git a/src/Framework/MockObject/Invocation/ObjectInvocation.php b/src/Framework/MockObject/Invocation/ObjectInvocation.php
index 5046a178727..20eb8a69dde 100644
--- a/src/Framework/MockObject/Invocation/ObjectInvocation.php
+++ b/src/Framework/MockObject/Invocation/ObjectInvocation.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework\MockObject\Invocation;
 
 /**
- * Represents a non-static invocation.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ObjectInvocation extends StaticInvocation
 {
diff --git a/src/Framework/MockObject/Invocation/StaticInvocation.php b/src/Framework/MockObject/Invocation/StaticInvocation.php
index fd9caf4177e..184181a6611 100644
--- a/src/Framework/MockObject/Invocation/StaticInvocation.php
+++ b/src/Framework/MockObject/Invocation/StaticInvocation.php
@@ -16,7 +16,7 @@
 use SebastianBergmann\Exporter\Exporter;
 
 /**
- * Represents a static invocation.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class StaticInvocation implements Invocation, SelfDescribing
 {
diff --git a/src/Framework/MockObject/InvocationMocker.php b/src/Framework/MockObject/InvocationMocker.php
index e1163b63688..3a65c4b6657 100644
--- a/src/Framework/MockObject/InvocationMocker.php
+++ b/src/Framework/MockObject/InvocationMocker.php
@@ -19,11 +19,7 @@
 use PHPUnit\Framework\MockObject\Stub\MatcherCollection;
 
 /**
- * Mocker for invocations which are sent from
- * MockObject objects.
- *
- * Keeps track of all expectations and stubs as well as registering
- * identifications for builders.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class InvocationMocker implements MatcherCollection, Invokable, NamespaceMatch
 {
diff --git a/src/Framework/MockObject/Invokable.php b/src/Framework/MockObject/Invokable.php
index 6ffa2b4d5a4..903880cae41 100644
--- a/src/Framework/MockObject/Invokable.php
+++ b/src/Framework/MockObject/Invokable.php
@@ -10,10 +10,7 @@
 namespace PHPUnit\Framework\MockObject;
 
 /**
- * Interface for classes which can be invoked.
- *
- * The invocation will be taken from a mock object and passed to an object
- * of this class.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Invokable extends Verifiable
 {
diff --git a/src/Framework/MockObject/Matcher.php b/src/Framework/MockObject/Matcher.php
index 41925371a5f..583d035985a 100644
--- a/src/Framework/MockObject/Matcher.php
+++ b/src/Framework/MockObject/Matcher.php
@@ -19,13 +19,7 @@
 use PHPUnit\Framework\TestFailure;
 
 /**
- * Main matcher which defines a full expectation using method, parameter and
- * invocation matchers.
- * This matcher encapsulates all the other matchers and allows the builder to
- * set the specific matchers when the appropriate methods are called (once(),
- * where() etc.).
- *
- * All properties are public so that they can easily be accessed by the builder.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Matcher implements MatcherInvocation
 {
diff --git a/src/Framework/MockObject/Matcher/AnyInvokedCount.php b/src/Framework/MockObject/Matcher/AnyInvokedCount.php
index 2094280a48e..15504ac9a37 100644
--- a/src/Framework/MockObject/Matcher/AnyInvokedCount.php
+++ b/src/Framework/MockObject/Matcher/AnyInvokedCount.php
@@ -10,8 +10,7 @@
 namespace PHPUnit\Framework\MockObject\Matcher;
 
 /**
- * Invocation matcher which checks if a method has been invoked zero or more
- * times. This matcher will always match.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class AnyInvokedCount extends InvokedRecorder
 {
diff --git a/src/Framework/MockObject/Matcher/AnyParameters.php b/src/Framework/MockObject/Matcher/AnyParameters.php
index 2b2b764ab9e..24386951624 100644
--- a/src/Framework/MockObject/Matcher/AnyParameters.php
+++ b/src/Framework/MockObject/Matcher/AnyParameters.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\MockObject\Invocation as BaseInvocation;
 
 /**
- * Invocation matcher which allows any parameters to a method.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class AnyParameters extends StatelessInvocation
 {
diff --git a/src/Framework/MockObject/Matcher/ConsecutiveParameters.php b/src/Framework/MockObject/Matcher/ConsecutiveParameters.php
index 4114b2907f3..bd44091a6c4 100644
--- a/src/Framework/MockObject/Matcher/ConsecutiveParameters.php
+++ b/src/Framework/MockObject/Matcher/ConsecutiveParameters.php
@@ -15,14 +15,7 @@
 use PHPUnit\Framework\MockObject\Invocation as BaseInvocation;
 
 /**
- * Invocation matcher which looks for sets of specific parameters in the invocations.
- *
- * Checks the parameters of the incoming invocations, the parameter list is
- * checked against the defined constraints in $parameters. If the constraint
- * is met it will return true in matches().
- *
- * It takes a list of match groups and and increases a call index after each invocation.
- * So the first invocation uses the first group of constraints, the second the next and so on.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ConsecutiveParameters extends StatelessInvocation
 {
diff --git a/src/Framework/MockObject/Matcher/DeferredError.php b/src/Framework/MockObject/Matcher/DeferredError.php
index 4c1296acb07..160aac497bc 100644
--- a/src/Framework/MockObject/Matcher/DeferredError.php
+++ b/src/Framework/MockObject/Matcher/DeferredError.php
@@ -11,6 +11,9 @@
 
 use PHPUnit\Framework\MockObject\Invocation as BaseInvocation;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class DeferredError extends StatelessInvocation
 {
     /**
diff --git a/src/Framework/MockObject/Matcher/Invocation.php b/src/Framework/MockObject/Matcher/Invocation.php
index 99e47617684..bb4f959eff3 100644
--- a/src/Framework/MockObject/Matcher/Invocation.php
+++ b/src/Framework/MockObject/Matcher/Invocation.php
@@ -14,8 +14,7 @@
 use PHPUnit\Framework\SelfDescribing;
 
 /**
- * Interface for classes which matches an invocation based on its
- * method name, argument, order or call count.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Invocation extends SelfDescribing, Verifiable
 {
diff --git a/src/Framework/MockObject/Matcher/InvokedAtIndex.php b/src/Framework/MockObject/Matcher/InvokedAtIndex.php
index e437f0decaf..796b04ec4b2 100644
--- a/src/Framework/MockObject/Matcher/InvokedAtIndex.php
+++ b/src/Framework/MockObject/Matcher/InvokedAtIndex.php
@@ -13,14 +13,7 @@
 use PHPUnit\Framework\MockObject\Invocation as BaseInvocation;
 
 /**
- * Invocation matcher which checks if a method was invoked at a certain index.
- *
- * If the expected index number does not match the current invocation index it
- * will not match which means it skips all method and parameter matching. Only
- * once the index is reached will the method and parameter start matching and
- * verifying.
- *
- * If the index is never reached it will throw an exception in index.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class InvokedAtIndex implements Invocation
 {
diff --git a/src/Framework/MockObject/Matcher/InvokedAtLeastCount.php b/src/Framework/MockObject/Matcher/InvokedAtLeastCount.php
index 3f882d787eb..5af382efe07 100644
--- a/src/Framework/MockObject/Matcher/InvokedAtLeastCount.php
+++ b/src/Framework/MockObject/Matcher/InvokedAtLeastCount.php
@@ -12,8 +12,7 @@
 use PHPUnit\Framework\ExpectationFailedException;
 
 /**
- * Invocation matcher which checks if a method has been invoked at least
- * N times.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class InvokedAtLeastCount extends InvokedRecorder
 {
diff --git a/src/Framework/MockObject/Matcher/InvokedAtLeastOnce.php b/src/Framework/MockObject/Matcher/InvokedAtLeastOnce.php
index f5ecaf32cc6..cb0d4fde379 100644
--- a/src/Framework/MockObject/Matcher/InvokedAtLeastOnce.php
+++ b/src/Framework/MockObject/Matcher/InvokedAtLeastOnce.php
@@ -12,10 +12,7 @@
 use PHPUnit\Framework\ExpectationFailedException;
 
 /**
- * Invocation matcher which checks if a method has been invoked at least one
- * time.
- *
- * If the number of invocations is 0 it will throw an exception in verify.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class InvokedAtLeastOnce extends InvokedRecorder
 {
diff --git a/src/Framework/MockObject/Matcher/InvokedAtMostCount.php b/src/Framework/MockObject/Matcher/InvokedAtMostCount.php
index bb3d31a7d63..40fec094017 100644
--- a/src/Framework/MockObject/Matcher/InvokedAtMostCount.php
+++ b/src/Framework/MockObject/Matcher/InvokedAtMostCount.php
@@ -12,8 +12,7 @@
 use PHPUnit\Framework\ExpectationFailedException;
 
 /**
- * Invocation matcher which checks if a method has been invoked at least
- * N times.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class InvokedAtMostCount extends InvokedRecorder
 {
diff --git a/src/Framework/MockObject/Matcher/InvokedCount.php b/src/Framework/MockObject/Matcher/InvokedCount.php
index 9c959d37108..0106542e4e9 100644
--- a/src/Framework/MockObject/Matcher/InvokedCount.php
+++ b/src/Framework/MockObject/Matcher/InvokedCount.php
@@ -13,12 +13,7 @@
 use PHPUnit\Framework\MockObject\Invocation as BaseInvocation;
 
 /**
- * Invocation matcher which checks if a method has been invoked a certain amount
- * of times.
- * If the number of invocations exceeds the value it will immediately throw an
- * exception,
- * If the number is less it will later be checked in verify() and also throw an
- * exception.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class InvokedCount extends InvokedRecorder
 {
diff --git a/src/Framework/MockObject/Matcher/InvokedRecorder.php b/src/Framework/MockObject/Matcher/InvokedRecorder.php
index 07a84f63bf5..3de42afa474 100644
--- a/src/Framework/MockObject/Matcher/InvokedRecorder.php
+++ b/src/Framework/MockObject/Matcher/InvokedRecorder.php
@@ -12,10 +12,7 @@
 use PHPUnit\Framework\MockObject\Invocation as BaseInvocation;
 
 /**
- * Records invocations and provides convenience methods for checking them later
- * on.
- * This abstract class can be implemented by matchers which needs to check the
- * number of times an invocation has occurred.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 abstract class InvokedRecorder implements Invocation
 {
diff --git a/src/Framework/MockObject/Matcher/MethodName.php b/src/Framework/MockObject/Matcher/MethodName.php
index 3cb9ba1099d..5eda98d139d 100644
--- a/src/Framework/MockObject/Matcher/MethodName.php
+++ b/src/Framework/MockObject/Matcher/MethodName.php
@@ -15,11 +15,7 @@
 use PHPUnit\Util\InvalidArgumentHelper;
 
 /**
- * Invocation matcher which looks for a specific method name in the invocations.
- *
- * Checks the method name all incoming invocations, the name is checked against
- * the defined constraint $constraint. If the constraint is met it will return
- * true in matches().
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class MethodName extends StatelessInvocation
 {
diff --git a/src/Framework/MockObject/Matcher/Parameters.php b/src/Framework/MockObject/Matcher/Parameters.php
index 14802089fcb..143f7857c81 100644
--- a/src/Framework/MockObject/Matcher/Parameters.php
+++ b/src/Framework/MockObject/Matcher/Parameters.php
@@ -16,11 +16,7 @@
 use PHPUnit\Framework\MockObject\Invocation as BaseInvocation;
 
 /**
- * Invocation matcher which looks for specific parameters in the invocations.
- *
- * Checks the parameters of all incoming invocations, the parameter list is
- * checked against the defined constraints in $parameters. If the constraint
- * is met it will return true in matches().
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Parameters extends StatelessInvocation
 {
diff --git a/src/Framework/MockObject/Matcher/StatelessInvocation.php b/src/Framework/MockObject/Matcher/StatelessInvocation.php
index e2ef4ad4830..9100b135179 100644
--- a/src/Framework/MockObject/Matcher/StatelessInvocation.php
+++ b/src/Framework/MockObject/Matcher/StatelessInvocation.php
@@ -12,11 +12,7 @@
 use PHPUnit\Framework\MockObject\Invocation as BaseInvocation;
 
 /**
- * Invocation matcher which does not care about previous state from earlier
- * invocations.
- *
- * This abstract class can be implemented by matchers which does not care about
- * state but only the current run-time value of the invocation itself.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 abstract class StatelessInvocation implements Invocation
 {
diff --git a/src/Framework/MockObject/MockBuilder.php b/src/Framework/MockObject/MockBuilder.php
index dd221f1cf19..cfe6e7f1424 100644
--- a/src/Framework/MockObject/MockBuilder.php
+++ b/src/Framework/MockObject/MockBuilder.php
@@ -11,9 +11,6 @@
 
 use PHPUnit\Framework\TestCase;
 
-/**
- * Implementation of the Builder pattern for Mock objects.
- */
 final class MockBuilder
 {
     /**
diff --git a/src/Framework/MockObject/MockMethod.php b/src/Framework/MockObject/MockMethod.php
index 213e2cc9b90..c380a6ea5c0 100644
--- a/src/Framework/MockObject/MockMethod.php
+++ b/src/Framework/MockObject/MockMethod.php
@@ -14,6 +14,9 @@
 use ReflectionMethod;
 use Text_Template;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class MockMethod
 {
     /**
diff --git a/src/Framework/MockObject/MockMethodSet.php b/src/Framework/MockObject/MockMethodSet.php
index b81536a73c1..422019f79e7 100644
--- a/src/Framework/MockObject/MockMethodSet.php
+++ b/src/Framework/MockObject/MockMethodSet.php
@@ -10,6 +10,9 @@
  */
 namespace PHPUnit\Framework\MockObject;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class MockMethodSet
 {
     /**
diff --git a/src/Framework/MockObject/MockObject.php b/src/Framework/MockObject/MockObject.php
index cbb13676413..0a94beee9ae 100644
--- a/src/Framework/MockObject/MockObject.php
+++ b/src/Framework/MockObject/MockObject.php
@@ -14,9 +14,6 @@
 use PHPUnit\Framework\MockObject\Matcher\Invocation;
 
 /**
- * Interface for all mock objects which are generated by
- * MockBuilder.
- *
  * @method InvocationMocker method($constraint)
  */
 interface MockObject /*extends Verifiable*/
diff --git a/src/Framework/MockObject/Stub.php b/src/Framework/MockObject/Stub.php
index b542f58b3ea..f14d4014a0c 100644
--- a/src/Framework/MockObject/Stub.php
+++ b/src/Framework/MockObject/Stub.php
@@ -12,10 +12,7 @@
 use PHPUnit\Framework\SelfDescribing;
 
 /**
- * An object that stubs the process of a normal method for a mock object.
- *
- * The stub object will replace the code for the stubbed method and return a
- * specific value instead of the original value.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Stub extends SelfDescribing
 {
diff --git a/src/Framework/MockObject/Stub/ConsecutiveCalls.php b/src/Framework/MockObject/Stub/ConsecutiveCalls.php
index 5253dfe6ebe..8806a4700e2 100644
--- a/src/Framework/MockObject/Stub/ConsecutiveCalls.php
+++ b/src/Framework/MockObject/Stub/ConsecutiveCalls.php
@@ -14,7 +14,7 @@
 use SebastianBergmann\Exporter\Exporter;
 
 /**
- * Stubs a method by returning a user-defined stack of values.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ConsecutiveCalls implements Stub
 {
diff --git a/src/Framework/MockObject/Stub/Exception.php b/src/Framework/MockObject/Stub/Exception.php
index 4651012069f..a8f0eb2a119 100644
--- a/src/Framework/MockObject/Stub/Exception.php
+++ b/src/Framework/MockObject/Stub/Exception.php
@@ -14,7 +14,7 @@
 use SebastianBergmann\Exporter\Exporter;
 
 /**
- * Stubs a method by raising a user-defined exception.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Exception implements Stub
 {
diff --git a/src/Framework/MockObject/Stub/MatcherCollection.php b/src/Framework/MockObject/Stub/MatcherCollection.php
index eb0aef4b610..94438989c08 100644
--- a/src/Framework/MockObject/Stub/MatcherCollection.php
+++ b/src/Framework/MockObject/Stub/MatcherCollection.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\MockObject\Matcher\Invocation;
 
 /**
- * Stubs a method by returning a user-defined value.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface MatcherCollection
 {
diff --git a/src/Framework/MockObject/Stub/ReturnArgument.php b/src/Framework/MockObject/Stub/ReturnArgument.php
index bc309bfe228..8f0aba48535 100644
--- a/src/Framework/MockObject/Stub/ReturnArgument.php
+++ b/src/Framework/MockObject/Stub/ReturnArgument.php
@@ -13,7 +13,7 @@
 use PHPUnit\Framework\MockObject\Stub;
 
 /**
- * Stubs a method by returning an argument that was passed to the mocked method.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ReturnArgument implements Stub
 {
diff --git a/src/Framework/MockObject/Stub/ReturnCallback.php b/src/Framework/MockObject/Stub/ReturnCallback.php
index 0adf252535d..cfa96c5f03f 100644
--- a/src/Framework/MockObject/Stub/ReturnCallback.php
+++ b/src/Framework/MockObject/Stub/ReturnCallback.php
@@ -12,6 +12,9 @@
 use PHPUnit\Framework\MockObject\Invocation;
 use PHPUnit\Framework\MockObject\Stub;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class ReturnCallback implements Stub
 {
     private $callback;
diff --git a/src/Framework/MockObject/Stub/ReturnReference.php b/src/Framework/MockObject/Stub/ReturnReference.php
index fa8ab6550c0..2ae692575eb 100644
--- a/src/Framework/MockObject/Stub/ReturnReference.php
+++ b/src/Framework/MockObject/Stub/ReturnReference.php
@@ -14,7 +14,7 @@
 use SebastianBergmann\Exporter\Exporter;
 
 /**
- * Stubs a method by returning a user-defined reference to a value.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ReturnReference implements Stub
 {
diff --git a/src/Framework/MockObject/Stub/ReturnSelf.php b/src/Framework/MockObject/Stub/ReturnSelf.php
index 165c5025f2f..2842fd54cde 100644
--- a/src/Framework/MockObject/Stub/ReturnSelf.php
+++ b/src/Framework/MockObject/Stub/ReturnSelf.php
@@ -15,7 +15,7 @@
 use PHPUnit\Framework\MockObject\Stub;
 
 /**
- * Stubs a method by returning the current object.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ReturnSelf implements Stub
 {
diff --git a/src/Framework/MockObject/Stub/ReturnStub.php b/src/Framework/MockObject/Stub/ReturnStub.php
index e4f1acabac3..81af06425b0 100644
--- a/src/Framework/MockObject/Stub/ReturnStub.php
+++ b/src/Framework/MockObject/Stub/ReturnStub.php
@@ -14,7 +14,7 @@
 use SebastianBergmann\Exporter\Exporter;
 
 /**
- * Stubs a method by returning a user-defined value.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ReturnStub implements Stub
 {
diff --git a/src/Framework/MockObject/Stub/ReturnValueMap.php b/src/Framework/MockObject/Stub/ReturnValueMap.php
index f61ef44953b..9df2995a54e 100644
--- a/src/Framework/MockObject/Stub/ReturnValueMap.php
+++ b/src/Framework/MockObject/Stub/ReturnValueMap.php
@@ -13,7 +13,7 @@
 use PHPUnit\Framework\MockObject\Stub;
 
 /**
- * Stubs a method by returning a value from a map.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ReturnValueMap implements Stub
 {
diff --git a/src/Framework/MockObject/Verifiable.php b/src/Framework/MockObject/Verifiable.php
index f9b9d51ad13..76aa2209e3a 100644
--- a/src/Framework/MockObject/Verifiable.php
+++ b/src/Framework/MockObject/Verifiable.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\ExpectationFailedException;
 
 /**
- * Interface for classes which must verify a given expectation.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface Verifiable
 {
diff --git a/src/Framework/OutputError.php b/src/Framework/OutputError.php
index 880d01a1905..1c8b37e5617 100644
--- a/src/Framework/OutputError.php
+++ b/src/Framework/OutputError.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class OutputError extends AssertionFailedError
 {
 }
diff --git a/src/Framework/PHPTAssertionFailedError.php b/src/Framework/PHPTAssertionFailedError.php
index 6a3fb18489b..b9dd13f655b 100644
--- a/src/Framework/PHPTAssertionFailedError.php
+++ b/src/Framework/PHPTAssertionFailedError.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class PHPTAssertionFailedError extends SyntheticError
 {
 }
diff --git a/src/Framework/RiskyTest.php b/src/Framework/RiskyTest.php
index 8ed64fb5878..c8be3aa987f 100644
--- a/src/Framework/RiskyTest.php
+++ b/src/Framework/RiskyTest.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 interface RiskyTest
 {
 }
diff --git a/src/Framework/RiskyTestError.php b/src/Framework/RiskyTestError.php
index 0f31f04281a..aa77b162e3b 100644
--- a/src/Framework/RiskyTestError.php
+++ b/src/Framework/RiskyTestError.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 class RiskyTestError extends AssertionFailedError implements RiskyTest
 {
 }
diff --git a/src/Framework/SelfDescribing.php b/src/Framework/SelfDescribing.php
index 218ea68e8be..73034f650ab 100644
--- a/src/Framework/SelfDescribing.php
+++ b/src/Framework/SelfDescribing.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * Interface for classes that can return a description of itself.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 interface SelfDescribing
 {
diff --git a/src/Framework/SkippedTest.php b/src/Framework/SkippedTest.php
index 533a7439419..c5ac84e6790 100644
--- a/src/Framework/SkippedTest.php
+++ b/src/Framework/SkippedTest.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 interface SkippedTest
 {
 }
diff --git a/src/Framework/SkippedTestCase.php b/src/Framework/SkippedTestCase.php
index 4c0a8004e07..3595ea387e0 100644
--- a/src/Framework/SkippedTestCase.php
+++ b/src/Framework/SkippedTestCase.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * A skipped test case
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class SkippedTestCase extends TestCase
 {
diff --git a/src/Framework/SkippedTestError.php b/src/Framework/SkippedTestError.php
index 3d57a82a298..7d553dcf3f1 100644
--- a/src/Framework/SkippedTestError.php
+++ b/src/Framework/SkippedTestError.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class SkippedTestError extends AssertionFailedError implements SkippedTest
 {
 }
diff --git a/src/Framework/SkippedTestSuiteError.php b/src/Framework/SkippedTestSuiteError.php
index 00bdfaea0ec..5448508a1ea 100644
--- a/src/Framework/SkippedTestSuiteError.php
+++ b/src/Framework/SkippedTestSuiteError.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class SkippedTestSuiteError extends AssertionFailedError implements SkippedTest
 {
 }
diff --git a/src/Framework/SyntheticError.php b/src/Framework/SyntheticError.php
index 648743db88c..c3124ba0c74 100644
--- a/src/Framework/SyntheticError.php
+++ b/src/Framework/SyntheticError.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * Creates a synthetic failed assertion.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class SyntheticError extends AssertionFailedError
 {
diff --git a/src/Framework/SyntheticSkippedError.php b/src/Framework/SyntheticSkippedError.php
index 8392d35f9e1..f6e155d7b9e 100644
--- a/src/Framework/SyntheticSkippedError.php
+++ b/src/Framework/SyntheticSkippedError.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Framework;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class SyntheticSkippedError extends SyntheticError implements SkippedTest
 {
 }
diff --git a/src/Framework/TestFailure.php b/src/Framework/TestFailure.php
index 8a18cdaa29e..1722d66603b 100644
--- a/src/Framework/TestFailure.php
+++ b/src/Framework/TestFailure.php
@@ -13,7 +13,7 @@
 use Throwable;
 
 /**
- * A TestFailure collects a failed test together with the caught exception.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class TestFailure
 {
diff --git a/src/Framework/TestResult.php b/src/Framework/TestResult.php
index 99f5d251760..943bba2882b 100644
--- a/src/Framework/TestResult.php
+++ b/src/Framework/TestResult.php
@@ -28,7 +28,7 @@
 use Throwable;
 
 /**
- * A TestResult collects the results of executing a test case.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class TestResult implements Countable
 {
diff --git a/src/Framework/TestSuite.php b/src/Framework/TestSuite.php
index 6e103a81127..9639b2a4c80 100644
--- a/src/Framework/TestSuite.php
+++ b/src/Framework/TestSuite.php
@@ -21,7 +21,7 @@
 use Throwable;
 
 /**
- * A TestSuite is a composite of Tests. It runs a collection of test cases.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class TestSuite implements Test, SelfDescribing, IteratorAggregate
 {
diff --git a/src/Framework/TestSuiteIterator.php b/src/Framework/TestSuiteIterator.php
index 6218257ef5e..4c3e7f77565 100644
--- a/src/Framework/TestSuiteIterator.php
+++ b/src/Framework/TestSuiteIterator.php
@@ -12,7 +12,7 @@
 use RecursiveIterator;
 
 /**
- * Iterator for test suites.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class TestSuiteIterator implements RecursiveIterator
 {
diff --git a/src/Framework/UnintentionallyCoveredCodeError.php b/src/Framework/UnintentionallyCoveredCodeError.php
index 351b665d8bb..fcd1d824964 100644
--- a/src/Framework/UnintentionallyCoveredCodeError.php
+++ b/src/Framework/UnintentionallyCoveredCodeError.php
@@ -10,8 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * Extension to PHPUnit\Framework\AssertionFailedError to mark the special
- * case of a test that unintentionally covers code.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class UnintentionallyCoveredCodeError extends RiskyTestError
 {
diff --git a/src/Framework/Warning.php b/src/Framework/Warning.php
index 7795500c32e..35e94493cf0 100644
--- a/src/Framework/Warning.php
+++ b/src/Framework/Warning.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * Thrown when there is a warning.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Warning extends Exception implements SelfDescribing
 {
diff --git a/src/Framework/WarningTestCase.php b/src/Framework/WarningTestCase.php
index 7ad0bcd47b3..85017a189a2 100644
--- a/src/Framework/WarningTestCase.php
+++ b/src/Framework/WarningTestCase.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Framework;
 
 /**
- * A warning.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class WarningTestCase extends TestCase
 {
diff --git a/src/Runner/BaseTestRunner.php b/src/Runner/BaseTestRunner.php
index d7dda6c021e..258d69fd228 100644
--- a/src/Runner/BaseTestRunner.php
+++ b/src/Runner/BaseTestRunner.php
@@ -17,7 +17,7 @@
 use SebastianBergmann\FileIterator\Facade as FileIteratorFacade;
 
 /**
- * Base class for all test runners.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 abstract class BaseTestRunner
 {
diff --git a/src/Runner/Exception.php b/src/Runner/Exception.php
index 1d6edcc8ac7..44705f5675e 100644
--- a/src/Runner/Exception.php
+++ b/src/Runner/Exception.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Runner;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Exception extends \RuntimeException implements \PHPUnit\Exception
 {
 }
diff --git a/src/Runner/Filter/ExcludeGroupFilterIterator.php b/src/Runner/Filter/ExcludeGroupFilterIterator.php
index 8e899d43acc..d8a8643684f 100644
--- a/src/Runner/Filter/ExcludeGroupFilterIterator.php
+++ b/src/Runner/Filter/ExcludeGroupFilterIterator.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Runner\Filter;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class ExcludeGroupFilterIterator extends GroupFilterIterator
 {
     protected function doAccept(string $hash): bool
diff --git a/src/Runner/Filter/Factory.php b/src/Runner/Filter/Factory.php
index bef61e2e400..4072ad23b06 100644
--- a/src/Runner/Filter/Factory.php
+++ b/src/Runner/Filter/Factory.php
@@ -15,6 +15,9 @@
 use PHPUnit\Framework\TestSuite;
 use ReflectionClass;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Factory
 {
     /**
diff --git a/src/Runner/Filter/GroupFilterIterator.php b/src/Runner/Filter/GroupFilterIterator.php
index 992564fe17c..7d5ad253a2f 100644
--- a/src/Runner/Filter/GroupFilterIterator.php
+++ b/src/Runner/Filter/GroupFilterIterator.php
@@ -13,6 +13,9 @@
 use RecursiveFilterIterator;
 use RecursiveIterator;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 abstract class GroupFilterIterator extends RecursiveFilterIterator
 {
     /**
diff --git a/src/Runner/Filter/IncludeGroupFilterIterator.php b/src/Runner/Filter/IncludeGroupFilterIterator.php
index cb67fd29dc2..5f004f9ad4e 100644
--- a/src/Runner/Filter/IncludeGroupFilterIterator.php
+++ b/src/Runner/Filter/IncludeGroupFilterIterator.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Runner\Filter;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class IncludeGroupFilterIterator extends GroupFilterIterator
 {
     protected function doAccept(string $hash): bool
diff --git a/src/Runner/Filter/NameFilterIterator.php b/src/Runner/Filter/NameFilterIterator.php
index 2d82c918edd..1c5babc94d1 100644
--- a/src/Runner/Filter/NameFilterIterator.php
+++ b/src/Runner/Filter/NameFilterIterator.php
@@ -15,6 +15,9 @@
 use RecursiveFilterIterator;
 use RecursiveIterator;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class NameFilterIterator extends RecursiveFilterIterator
 {
     /**
diff --git a/src/Runner/Hook/TestListenerAdapter.php b/src/Runner/Hook/TestListenerAdapter.php
index 02ed7006b44..17542cde01e 100644
--- a/src/Runner/Hook/TestListenerAdapter.php
+++ b/src/Runner/Hook/TestListenerAdapter.php
@@ -16,6 +16,9 @@
 use PHPUnit\Framework\Warning;
 use PHPUnit\Util\Test as TestUtil;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class TestListenerAdapter implements TestListener
 {
     /**
diff --git a/src/Runner/PhptTestCase.php b/src/Runner/PhptTestCase.php
index 9737fe626af..c2a77a75ac7 100644
--- a/src/Runner/PhptTestCase.php
+++ b/src/Runner/PhptTestCase.php
@@ -24,7 +24,7 @@
 use Throwable;
 
 /**
- * Runner for PHPT test cases.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class PhptTestCase implements Test, SelfDescribing
 {
diff --git a/src/Runner/ResultCacheExtension.php b/src/Runner/ResultCacheExtension.php
index cb9b451ec4d..f1724f97dc6 100644
--- a/src/Runner/ResultCacheExtension.php
+++ b/src/Runner/ResultCacheExtension.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Runner;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class ResultCacheExtension implements AfterSuccessfulTestHook, AfterSkippedTestHook, AfterRiskyTestHook, AfterIncompleteTestHook, AfterTestErrorHook, AfterTestWarningHook, AfterTestFailureHook, AfterLastTestHook
 {
     /**
diff --git a/src/Runner/StandardTestSuiteLoader.php b/src/Runner/StandardTestSuiteLoader.php
index 7ef09c89a16..3afa68c65ed 100644
--- a/src/Runner/StandardTestSuiteLoader.php
+++ b/src/Runner/StandardTestSuiteLoader.php
@@ -15,7 +15,7 @@
 use ReflectionClass;
 
 /**
- * The standard test suite loader.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class StandardTestSuiteLoader implements TestSuiteLoader
 {
diff --git a/src/Runner/TestResultCache.php b/src/Runner/TestResultCache.php
index b8162c093aa..836d94516fa 100644
--- a/src/Runner/TestResultCache.php
+++ b/src/Runner/TestResultCache.php
@@ -11,6 +11,9 @@
 
 use PHPUnit\Util\Filesystem;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class TestResultCache implements \Serializable, TestResultCacheInterface
 {
     /**
diff --git a/src/Runner/TestResultCacheInterface.php b/src/Runner/TestResultCacheInterface.php
index a62e901e32b..aae9ac35dd5 100644
--- a/src/Runner/TestResultCacheInterface.php
+++ b/src/Runner/TestResultCacheInterface.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Runner;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 interface TestResultCacheInterface
 {
     public function getState($testName): int;
diff --git a/src/Runner/TestSuiteSorter.php b/src/Runner/TestSuiteSorter.php
index 3122edd8d64..95346a1a961 100644
--- a/src/Runner/TestSuiteSorter.php
+++ b/src/Runner/TestSuiteSorter.php
@@ -14,6 +14,9 @@
 use PHPUnit\Framework\TestCase;
 use PHPUnit\Framework\TestSuite;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class TestSuiteSorter
 {
     /**
diff --git a/src/TextUI/ResultPrinter.php b/src/TextUI/ResultPrinter.php
index e88f805f68f..0a828386ac9 100644
--- a/src/TextUI/ResultPrinter.php
+++ b/src/TextUI/ResultPrinter.php
@@ -26,7 +26,7 @@
 use SebastianBergmann\Timer\Timer;
 
 /**
- * Prints the result of a TextUI TestRunner run.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class ResultPrinter extends Printer implements TestListener
 {
diff --git a/src/TextUI/TestRunner.php b/src/TextUI/TestRunner.php
index b4047ae234d..6df0a45556d 100644
--- a/src/TextUI/TestRunner.php
+++ b/src/TextUI/TestRunner.php
@@ -60,8 +60,7 @@
 use SebastianBergmann\Invoker\Invoker;
 
 /**
- * A TestRunner for the Command Line Interface (CLI)
- * PHP SAPI Module.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class TestRunner extends BaseTestRunner
 {
diff --git a/src/Util/Blacklist.php b/src/Util/Blacklist.php
index c8bbcce0111..3692d8f983b 100644
--- a/src/Util/Blacklist.php
+++ b/src/Util/Blacklist.php
@@ -32,7 +32,7 @@
 use Text_Template;
 
 /**
- * Utility class for blacklisting PHPUnit's own source code files.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Blacklist
 {
diff --git a/src/Util/Color.php b/src/Util/Color.php
index dde0f3f6990..b1905319032 100644
--- a/src/Util/Color.php
+++ b/src/Util/Color.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Util;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Color
 {
     private const WHITESPACE_MAP = [
diff --git a/src/Util/Configuration.php b/src/Util/Configuration.php
index e67f35e7cd9..552b8d428a1 100644
--- a/src/Util/Configuration.php
+++ b/src/Util/Configuration.php
@@ -18,143 +18,7 @@
 use SebastianBergmann\FileIterator\Facade as FileIteratorFacade;
 
 /**
- * Wrapper for the PHPUnit XML configuration file.
- *
- * Example XML configuration file:
- * <code>
- * <?xml version="1.0" encoding="utf-8" ?>
- *
- * <phpunit backupGlobals="false"
- *          backupStaticAttributes="false"
- *          bootstrap="/path/to/bootstrap.php"
- *          cacheResult="true"
- *          cacheResultFile=".phpunit.result.cache"
- *          cacheTokens="false"
- *          columns="80"
- *          colors="false"
- *          stderr="false"
- *          convertDeprecationsToExceptions="true"
- *          convertErrorsToExceptions="true"
- *          convertNoticesToExceptions="true"
- *          convertWarningsToExceptions="true"
- *          disableCodeCoverageIgnore="false"
- *          forceCoversAnnotation="false"
- *          processIsolation="false"
- *          stopOnDefect="false"
- *          stopOnError="false"
- *          stopOnFailure="false"
- *          stopOnWarning="false"
- *          stopOnIncomplete="false"
- *          stopOnRisky="false"
- *          stopOnSkipped="false"
- *          failOnWarning="false"
- *          failOnRisky="false"
- *          extensionsDirectory="tools/phpunit.d"
- *          printerClass="PHPUnit\TextUI\ResultPrinter"
- *          testSuiteLoaderClass="PHPUnit\Runner\StandardTestSuiteLoader"
- *          defaultTestSuite=""
- *          beStrictAboutChangesToGlobalState="false"
- *          beStrictAboutCoversAnnotation="false"
- *          beStrictAboutOutputDuringTests="false"
- *          beStrictAboutResourceUsageDuringSmallTests="false"
- *          beStrictAboutTestsThatDoNotTestAnything="false"
- *          beStrictAboutTodoAnnotatedTests="false"
- *          defaultTimeLimit="0"
- *          enforceTimeLimit="false"
- *          ignoreDeprecatedCodeUnitsFromCodeCoverage="false"
- *          timeoutForSmallTests="1"
- *          timeoutForMediumTests="10"
- *          timeoutForLargeTests="60"
- *          verbose="false"
- *          reverseDefectList="false"
- *          registerMockObjectsFromTestArgumentsRecursively="false"
- *          executionOrder="default"
- *          executionOrderDefects="default"
- *          resolveDependencies="false">
- *   <testsuites>
- *     <testsuite name="My Test Suite">
- *       <directory suffix="Test.php" phpVersion="5.3.0" phpVersionOperator=">=">/path/to/files</directory>
- *       <file phpVersion="5.3.0" phpVersionOperator=">=">/path/to/MyTest.php</file>
- *       <exclude>/path/to/files/exclude</exclude>
- *     </testsuite>
- *   </testsuites>
- *
- *   <groups>
- *     <include>
- *       <group>name</group>
- *     </include>
- *     <exclude>
- *       <group>name</group>
- *     </exclude>
- *   </groups>
- *
- *   <testdoxGroups>
- *     <include>
- *       <group>name</group>
- *     </include>
- *     <exclude>
- *       <group>name</group>
- *     </exclude>
- *   </testdoxGroups>
- *
- *   <filter>
- *     <whitelist addUncoveredFilesFromWhitelist="true"
- *                processUncoveredFilesFromWhitelist="false">
- *       <directory suffix=".php">/path/to/files</directory>
- *       <file>/path/to/file</file>
- *       <exclude>
- *         <directory suffix=".php">/path/to/files</directory>
- *         <file>/path/to/file</file>
- *       </exclude>
- *     </whitelist>
- *   </filter>
- *
- *   <listeners>
- *     <listener class="MyListener" file="/optional/path/to/MyListener.php">
- *       <arguments>
- *         <array>
- *           <element key="0">
- *             <string>Sebastian</string>
- *           </element>
- *         </array>
- *         <integer>22</integer>
- *         <string>April</string>
- *         <double>19.78</double>
- *         <null/>
- *         <object class="stdClass"/>
- *         <file>MyRelativeFile.php</file>
- *         <directory>MyRelativeDir</directory>
- *       </arguments>
- *     </listener>
- *   </listeners>
- *
- *   <logging>
- *     <log type="coverage-html" target="/tmp/report" lowUpperBound="50" highLowerBound="90"/>
- *     <log type="coverage-clover" target="/tmp/clover.xml"/>
- *     <log type="coverage-crap4j" target="/tmp/crap.xml" threshold="30"/>
- *     <log type="plain" target="/tmp/logfile.txt"/>
- *     <log type="teamcity" target="/tmp/logfile.txt"/>
- *     <log type="junit" target="/tmp/logfile.xml"/>
- *     <log type="testdox-html" target="/tmp/testdox.html"/>
- *     <log type="testdox-text" target="/tmp/testdox.txt"/>
- *     <log type="testdox-xml" target="/tmp/testdox.xml"/>
- *   </logging>
- *
- *   <php>
- *     <includePath>.</includePath>
- *     <ini name="foo" value="bar"/>
- *     <const name="foo" value="bar"/>
- *     <var name="foo" value="bar"/>
- *     <env name="foo" value="bar"/>
- *     <post name="foo" value="bar"/>
- *     <get name="foo" value="bar"/>
- *     <cookie name="foo" value="bar"/>
- *     <server name="foo" value="bar"/>
- *     <files name="foo" value="bar"/>
- *     <request name="foo" value="bar"/>
- *   </php>
- * </phpunit>
- * </code>
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Configuration
 {
diff --git a/src/Util/ConfigurationGenerator.php b/src/Util/ConfigurationGenerator.php
index fe54b96fd80..0414d9881fe 100644
--- a/src/Util/ConfigurationGenerator.php
+++ b/src/Util/ConfigurationGenerator.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Util;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class ConfigurationGenerator
 {
     /**
diff --git a/src/Util/ErrorHandler.php b/src/Util/ErrorHandler.php
index f0ad8f9ba1d..7c7c6565e7d 100644
--- a/src/Util/ErrorHandler.php
+++ b/src/Util/ErrorHandler.php
@@ -15,7 +15,7 @@
 use PHPUnit\Framework\Error\Warning;
 
 /**
- * Error handler that converts PHP errors and warnings to exceptions.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class ErrorHandler
 {
diff --git a/src/Util/FileLoader.php b/src/Util/FileLoader.php
index 25c48af5762..817ee70d699 100644
--- a/src/Util/FileLoader.php
+++ b/src/Util/FileLoader.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\Exception;
 
 /**
- * Utility methods to load PHP sourcefiles.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class FileLoader
 {
diff --git a/src/Util/Filesystem.php b/src/Util/Filesystem.php
index 8b4fcc67c6d..8207a4fffc4 100644
--- a/src/Util/Filesystem.php
+++ b/src/Util/Filesystem.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Util;
 
 /**
- * Filesystem helpers.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Filesystem
 {
diff --git a/src/Util/Filter.php b/src/Util/Filter.php
index 1c25943f791..ee66efd3726 100644
--- a/src/Util/Filter.php
+++ b/src/Util/Filter.php
@@ -12,6 +12,9 @@
 use PHPUnit\Framework\Exception;
 use PHPUnit\Framework\SyntheticError;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Filter
 {
     /**
diff --git a/src/Util/Getopt.php b/src/Util/Getopt.php
index 21e8a9c4d1a..4a0522f4c3e 100644
--- a/src/Util/Getopt.php
+++ b/src/Util/Getopt.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\Exception;
 
 /**
- * Command-line options parsing class.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class Getopt
 {
diff --git a/src/Util/GlobalState.php b/src/Util/GlobalState.php
index 26ca3eb3639..6168dfb057e 100644
--- a/src/Util/GlobalState.php
+++ b/src/Util/GlobalState.php
@@ -11,6 +11,9 @@
 
 use Closure;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class GlobalState
 {
     /**
diff --git a/src/Util/Help.php b/src/Util/Help.php
index be212540f5a..7f3af5e1bed 100644
--- a/src/Util/Help.php
+++ b/src/Util/Help.php
@@ -12,6 +12,9 @@
 use PHPUnit\Util\Color;
 use SebastianBergmann\Environment\Console;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Help
 {
     private const LEFT_MARGIN = '  ';
diff --git a/src/Util/InvalidArgumentHelper.php b/src/Util/InvalidArgumentHelper.php
index 832e6bf7135..185f649addf 100644
--- a/src/Util/InvalidArgumentHelper.php
+++ b/src/Util/InvalidArgumentHelper.php
@@ -12,8 +12,7 @@
 use PHPUnit\Framework\Exception;
 
 /**
- * Factory for PHPUnit\Framework\Exception objects that are used to describe
- * invalid arguments passed to a function or method.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class InvalidArgumentHelper
 {
diff --git a/src/Util/Json.php b/src/Util/Json.php
index 6a086cd169c..458dc31403b 100644
--- a/src/Util/Json.php
+++ b/src/Util/Json.php
@@ -11,6 +11,9 @@
 
 use PHPUnit\Framework\Exception;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Json
 {
     /**
diff --git a/src/Util/Log/JUnit.php b/src/Util/Log/JUnit.php
index 07869e0af1c..02c419ed52a 100644
--- a/src/Util/Log/JUnit.php
+++ b/src/Util/Log/JUnit.php
@@ -26,9 +26,7 @@
 use ReflectionException;
 
 /**
- * A TestListener that generates a logfile of the test execution in XML markup.
- *
- * The XML markup used is the same as the one that is used by the JUnit Ant task.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class JUnit extends Printer implements TestListener
 {
diff --git a/src/Util/Log/TeamCity.php b/src/Util/Log/TeamCity.php
index 6e8af3105d8..250d471f3ff 100644
--- a/src/Util/Log/TeamCity.php
+++ b/src/Util/Log/TeamCity.php
@@ -24,8 +24,7 @@
 use SebastianBergmann\Comparator\ComparisonFailure;
 
 /**
- * A TestListener that generates a logfile of the test execution using the
- * TeamCity format (for use with PhpStorm, for instance).
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class TeamCity extends ResultPrinter
 {
diff --git a/src/Util/NullTestResultCache.php b/src/Util/NullTestResultCache.php
index 2076f2b3921..f5e807bbe96 100644
--- a/src/Util/NullTestResultCache.php
+++ b/src/Util/NullTestResultCache.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Runner;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class NullTestResultCache implements TestResultCacheInterface
 {
     public function getState($testName): int
diff --git a/src/Util/PHP/AbstractPhpProcess.php b/src/Util/PHP/AbstractPhpProcess.php
index 8f42287fea5..f0c0887e581 100644
--- a/src/Util/PHP/AbstractPhpProcess.php
+++ b/src/Util/PHP/AbstractPhpProcess.php
@@ -20,7 +20,7 @@
 use SebastianBergmann\Environment\Runtime;
 
 /**
- * Utility methods for PHP sub-processes.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 abstract class AbstractPhpProcess
 {
diff --git a/src/Util/PHP/DefaultPhpProcess.php b/src/Util/PHP/DefaultPhpProcess.php
index 0fc71fb61b5..3a8797086d0 100644
--- a/src/Util/PHP/DefaultPhpProcess.php
+++ b/src/Util/PHP/DefaultPhpProcess.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\Exception;
 
 /**
- * Default utility for PHP sub-processes.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class DefaultPhpProcess extends AbstractPhpProcess
 {
diff --git a/src/Util/PHP/WindowsPhpProcess.php b/src/Util/PHP/WindowsPhpProcess.php
index c839a5888f9..14831fa3b9e 100644
--- a/src/Util/PHP/WindowsPhpProcess.php
+++ b/src/Util/PHP/WindowsPhpProcess.php
@@ -12,11 +12,7 @@
 use PHPUnit\Framework\Exception;
 
 /**
- * Windows utility for PHP sub-processes.
- *
- * Reading from STDOUT or STDERR hangs forever on Windows if the output is
- * too large.
- *
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  * @see https://bugs.php.net/bug.php?id=51800
  */
 final class WindowsPhpProcess extends DefaultPhpProcess
diff --git a/src/Util/Printer.php b/src/Util/Printer.php
index d759c1d1add..57c32dce517 100644
--- a/src/Util/Printer.php
+++ b/src/Util/Printer.php
@@ -12,7 +12,7 @@
 use PHPUnit\Framework\Exception;
 
 /**
- * Utility class that can print to STDOUT or write to a file.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class Printer
 {
diff --git a/src/Util/RegularExpression.php b/src/Util/RegularExpression.php
index 92956d8b552..303f4b287ab 100644
--- a/src/Util/RegularExpression.php
+++ b/src/Util/RegularExpression.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Util;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class RegularExpression
 {
     /**
diff --git a/src/Util/Test.php b/src/Util/Test.php
index 36b71e15954..9286b0998f7 100644
--- a/src/Util/Test.php
+++ b/src/Util/Test.php
@@ -27,6 +27,9 @@
 use SebastianBergmann\Environment\OperatingSystem;
 use Traversable;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Test
 {
     /**
diff --git a/src/Util/TestDox/CliTestDoxPrinter.php b/src/Util/TestDox/CliTestDoxPrinter.php
index 2fbd7b35bd0..5d3197558ae 100644
--- a/src/Util/TestDox/CliTestDoxPrinter.php
+++ b/src/Util/TestDox/CliTestDoxPrinter.php
@@ -18,8 +18,7 @@
 use SebastianBergmann\Timer\Timer;
 
 /**
- * This printer is for CLI output only. For the classes that output to file, html and xml,
- * please refer to the PHPUnit\Util\TestDox namespace
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 class CliTestDoxPrinter extends TestDoxPrinter
 {
diff --git a/src/Util/TestDox/HtmlResultPrinter.php b/src/Util/TestDox/HtmlResultPrinter.php
index 426c180612d..1beb8be473d 100644
--- a/src/Util/TestDox/HtmlResultPrinter.php
+++ b/src/Util/TestDox/HtmlResultPrinter.php
@@ -10,7 +10,7 @@
 namespace PHPUnit\Util\TestDox;
 
 /**
- * Prints TestDox documentation in HTML format.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class HtmlResultPrinter extends ResultPrinter
 {
diff --git a/src/Util/TestDox/NamePrettifier.php b/src/Util/TestDox/NamePrettifier.php
index 3d42eeb79d7..0c59a7d641f 100644
--- a/src/Util/TestDox/NamePrettifier.php
+++ b/src/Util/TestDox/NamePrettifier.php
@@ -14,7 +14,7 @@
 use SebastianBergmann\Exporter\Exporter;
 
 /**
- * Prettifies class and method names for use in TestDox documentation.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class NamePrettifier
 {
diff --git a/src/Util/TestDox/ResultPrinter.php b/src/Util/TestDox/ResultPrinter.php
index c1b2982e139..bb3a85fcc47 100644
--- a/src/Util/TestDox/ResultPrinter.php
+++ b/src/Util/TestDox/ResultPrinter.php
@@ -20,7 +20,7 @@
 use PHPUnit\Util\Printer;
 
 /**
- * Base class for printers of TestDox documentation.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 abstract class ResultPrinter extends Printer implements TestListener
 {
diff --git a/src/Util/TestDox/TestDoxPrinter.php b/src/Util/TestDox/TestDoxPrinter.php
index bd6b54ab334..095f3107de6 100644
--- a/src/Util/TestDox/TestDoxPrinter.php
+++ b/src/Util/TestDox/TestDoxPrinter.php
@@ -20,6 +20,9 @@
 use PHPUnit\Runner\TestSuiteSorter;
 use PHPUnit\TextUI\ResultPrinter;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 class TestDoxPrinter extends ResultPrinter
 {
     /**
diff --git a/src/Util/TestDox/TextResultPrinter.php b/src/Util/TestDox/TextResultPrinter.php
index 60ca0a33026..ee080039a78 100644
--- a/src/Util/TestDox/TextResultPrinter.php
+++ b/src/Util/TestDox/TextResultPrinter.php
@@ -10,8 +10,7 @@
 namespace PHPUnit\Util\TestDox;
 
 /**
- * Prints TestDox documentation in text format to files.
- * For the CLI testdox printer please refer to \PHPUnit\TextUI\TextDoxPrinter.
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
  */
 final class TextResultPrinter extends ResultPrinter
 {
diff --git a/src/Util/TestDox/XmlResultPrinter.php b/src/Util/TestDox/XmlResultPrinter.php
index 72cab8808a3..b161a11884b 100644
--- a/src/Util/TestDox/XmlResultPrinter.php
+++ b/src/Util/TestDox/XmlResultPrinter.php
@@ -21,6 +21,9 @@
 use PHPUnit\Util\Printer;
 use ReflectionClass;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class XmlResultPrinter extends Printer implements TestListener
 {
     /**
diff --git a/src/Util/TextTestListRenderer.php b/src/Util/TextTestListRenderer.php
index 1181a2206b6..500038044f0 100644
--- a/src/Util/TextTestListRenderer.php
+++ b/src/Util/TextTestListRenderer.php
@@ -13,6 +13,9 @@
 use PHPUnit\Framework\TestSuite;
 use PHPUnit\Runner\PhptTestCase;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class TextTestListRenderer
 {
     /**
diff --git a/src/Util/Type.php b/src/Util/Type.php
index ebe0ea0a443..ec6a1d78a4a 100644
--- a/src/Util/Type.php
+++ b/src/Util/Type.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Util;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Type
 {
     public static function isType(string $type): bool
diff --git a/src/Util/XdebugFilterScriptGenerator.php b/src/Util/XdebugFilterScriptGenerator.php
index 270e0714aaf..d87a89774b6 100644
--- a/src/Util/XdebugFilterScriptGenerator.php
+++ b/src/Util/XdebugFilterScriptGenerator.php
@@ -9,6 +9,9 @@
  */
 namespace PHPUnit\Util;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class XdebugFilterScriptGenerator
 {
     public function generate(array $filterData): string
diff --git a/src/Util/Xml.php b/src/Util/Xml.php
index 74490855b8e..b74acc1e6b2 100644
--- a/src/Util/Xml.php
+++ b/src/Util/Xml.php
@@ -17,6 +17,9 @@
 use PHPUnit\Framework\Exception;
 use ReflectionClass;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class Xml
 {
     public static function import(DOMElement $element): DOMElement
diff --git a/src/Util/XmlTestListRenderer.php b/src/Util/XmlTestListRenderer.php
index ee91af3d4e2..7d7e44a3649 100644
--- a/src/Util/XmlTestListRenderer.php
+++ b/src/Util/XmlTestListRenderer.php
@@ -13,6 +13,9 @@
 use PHPUnit\Framework\TestSuite;
 use PHPUnit\Runner\PhptTestCase;
 
+/**
+ * @internal This class is not covered by the backward compatibility promise for PHPUnit
+ */
 final class XmlTestListRenderer
 {
     /**