Generator for redirecting boot to offline update mode
/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 \m[blue]System Updates Specification\m[]\s-2\u[1]\d\s+2.
systemd-system-update-generator implements the \m[blue]generator specification\m[]\s-2\u[2]\d\s+2.
System Updates Specification
http://freedesktop.org/wiki/Software/systemd/SystemUpdates
generator specification
http://www.freedesktop.org/wiki/Software/systemd/Generators