systemd --user runs as a system service called
user@.service. This is a template service. Your instance might be
firstname.lastname@example.org. The number is your user ID (
The system shutdown procedure will signal
email@example.com to stop, with the usual signal
SIGTERM. In response, the
systemd --user process will signal each user service to stop.[*]
systemctl cat user@.service suggests there is little point setting a user service timeout (grace period) any longer than 120 seconds. After that, all the user services will recieve
SIGKILL from the main system (PID 1).
SIGKILL is immediately fatal.
To understand the settings in this file, see
man systemd.kill, and also
[*] Technically, what
systemd --user does in response to
SIGTERM is to activate the user unit
exit.target. This is mentioned in
man systemd. For more information about
systemctl cat exit.target will tell you to look in
man systemd.special :-).