What makes an app part of session?

[ilya korchemkin]

I'm using xfmpc and sometimes it's started automatically when I log
in, sometimes not. It seems that it's not started if it was minimized
to tray when I log out.  Other apps like Pidgin or Liferea are
restored not depending on this.
So, the question is, what should be in the app that makes it a part of
a session? Links to manuals or keywords for googling are welcomed.