The Server Reference Manual states that there are a number of reason for session termination:
“Socket closed” = Client-induced termination
“LRU” = Terminated as Least Recently Used in favor of new user
“Shutdown” = Server-induced termination for other reasons
But that this is not a complete list, as the exit value in some cases comes from the operating system.
Other common Exit Reasons are:
Session expired after idle time Session Idle Time exceeded the parameters set for the Document / Server Killed because Named User Cal was needed from another client Where the User is using two different machines / browsers to view the same document this "kills the session" to use for the other machine.