First lookup your release in config.map.
Each line contains properties of a release queue.
CMSDIST_TAG will tell you which CMSDIST tag / branch to lookup in:
https://github.com/cms-sw/cmsdist
Look up for the spec related to your external and you should find in the first rows either a line of the kind:
Source: <some-url>
for example:
Source: git+https://github.com/%github_user/root.git?obj=%{branch}/%{tag}&export=%{n}-%{realversion}&output=/%{n}-%{realversion}-%{tag}.tgz
the %{defined-variable} gets expanded to their value, as required by rpm. In particular in many cases we have:
%tag: the hash commit to be used for the external.%branch: the branch on which the commit its located.github_user: the user owning the repository to be used.