The previous session id Emacs got from session manager.
If Emacs is running on a window system that has a session manager, the
session manager gives Emacs a session id. It is feasible for Emacs Lisp
code to use the session id to save configuration in, for example, a file
with a file name based on the session id. If Emacs is running when the
window system is shut down, the session manager remembers that Emacs was
running and saves the session id Emacs had.
When the window system is started again, the session manager restarts
Emacs and hands Emacs the session id it had the last time it was
running. This is now the previous session id and the value of this
variable. If configuration was saved in a file as stated above, the
previous session id shall be used to reconstruct the file name.
The session id Emacs has while it is running is in the variable
`x-session-id'. The value of this variable and `x-session-id' may be the
same, depending on how the session manager works.
See also `emacs-save-session-functions', `emacs-session-save' and
`emacs-session-restore'."