What is SIGRTMIN+24 in syslog?
Solution 1:
Starting with some recent (not sure exactly) versions of systemd, a user systemd (systemd --user) instance is started (one per user) when a user is logging in. The systemd user instance can then start processes for the user.
When the user closes his last session, that user instance of systemd is killed by the user systemd-exit service (/usr/lib/systemd/user/systemd-exit.service).
So the shutdown sequence here is the shutdown of the systemd --user process