[I hesitated whether to post this in meta or here. But although this involves terminology, it is of general use, not only on this website.]
It looks like at least in Xfce 4.10 (with the session manager "Xfce4-session") this term means a desktop environment in the case in which different DE are installed beside Xfce; selecting between them is presented in the login window as selection between "sessions".
And then what is selected is the DE; selecting a session here is not selecting one among others of the same DE, but the DE itself
(In my case it says Xfce, LXDE, Openbox.)
What is selected above is the username and the DE, not a saved session.
But after making this selection, if in Settings/'Session and Startup'/General the option 'Display chooser on login' is checked, a new selection is possible between a different type of sessions, those that are DE-specific, a sort of sub-sessions, saved by a certain user within a certain DE, with the option under Settings/'Session and Startup'/Session/'Save session'.
Passwords and usernames are related to selecting and entering generic sessions (DE).
Is this normal and accepted use, or is it just an accident, maybe limited to Xfce?
(I have stumbled into this when searching for this workaround)
Edit after comments:
What I think it happened is that the proper use of the term 'session' being that of the set of options and running programs saved by the user in a certain DE, this meaning was diffused upstream to the initial selection of the user and DE. Both selections may be implicit (automated) if the user does not activate 'login password' and 'chooser'. But while many people want to be asked for password at login/startup, they do not want to select each time between saved sessions. As the first selection (user/DE/password) is the most common, it took the name "session" which now appears in the Xfce login window.
The term 'login' must have played a role in this because it suggests that a login must be one into a session. But if so, there must be two types of logins if there are two types of sessions. The term 'login' thus appears in Settings/'Users and Groups' when setting on being asked for password
but also under 'Session and Startup' regarding choosing (or not) between saved sessions - after the first 'login' has been made.
These are two different types of logins.
I was wandering whether behind this there's some consensus or is it just an accident. This may cause confusion especially to new users, whom these GUI settings are supposed to help - and especially when practical problems occur as seen in the linked answer. Trying to solve these problems and looking at the Xfce settings as they are displayed now, v.4.10, the double use of the term makes things worse.