/usr/lib/systemd/system-generators/systemd-system-update-generator
systemd-system-update-generator is a generator that automatically redirects the boot process to system-update.target, if /system-update exists. This is required to implement the logic explained in the systemd.offline-updates(7).
systemd-system-update-generator implements systemd.generator(7).
systemd(1), systemd.special(7)