node.js request session property

i try to learn, how to build an app with node.js, express and backbone. For user authentication i use the cookie and session middleware, like this

  app.use(express.cookieParser());
  app.use(express.session({secret: "SocialNet secret key", store: new MemoryStore()}));  

now, if a request comes in, so i can use session property

req.session  

i encounter with a code snippet from book

app.post('/login', function(req, res) {
  console.log('login request');
  var email = req.param('email', null);
  var password = req.param('password', null);

  if ( null == email || email.length < 1
      || null == password || password.length < 1 ) {
    res.send(400);
    return;
  }

  Account.login(email, password, function(success) {
    if ( !success ) {
      res.send(401);
      return;
    }
    console.log('login was successful');
    req.session.loggedIn = true;
    res.send(200);
  });
});  

look at req.session.loggedIn, where loggedIn property comes from? Can i add some own request session property to req.session property?