What is the session in Passport, versus session-store?

[smhk]

I make a simple web site like this ..

passport.use(new Strategy(
    {
        usernameField: 'email',
        passwordField: 'password',
        session: false (or true ?)
    },
  function(email, password, cb) { ...

also using express-session ...

var session = require('express-session') ...
var app = express()
app.use(sess)
app.use(passport.initialize())
app.use(passport.session())

You login in the usual way ..

app.post('/login', urlencodedParser,
    passport.authenticate('local', {
        failureRedirect: '/loginagain'
    }),
        function(req, res) {
            console.log("you are in ............")

and express-session makes sessions:

app.get('/', ensureLoggedIn("/loginpage"), function(req, res, next) {
	console.log(req.session)
	req.session.testString = 'abc'

I'm afraid I just don't understand passport's session option

1. I just can't find the doco on it - sorry

2. If you are using express-session anyway, is there any reason one would use session:true in passport?

3. If I do set it to "true" in my small apps, there appears to be no difference? Indeed: is it OK / best / essential to set it to true (or false ??) when using with express-session?

4. Indeed - can I just not bother about express-session and just use the session feature in passport ... or is it something different I don't understand?

5. What then is the usual use case for passport's session:true? (Perhaps it has nothing to do with typical www sites, and I'm way off base?)

Thanks a million - I honestly couldn't find this explained anywhere. Cheers