systemd-system-update-generator — Generator for redirecting boot to offline update mode
/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
                System
                Updates Specification.
                
systemd-system-update-generator
                implements the generator
                specification.