systemd-debug-generator — Generator for enabling a runtime debug shell and masking specific units at boot
/usr/lib/systemd/system-generators/systemd-debug-generator
systemd-debug-generator is
                a generator that reads the kernel command line and
                understands three options:
If the systemd.mask= option is
                specified and followed by a unit name, this unit is
                masked for the runtime, similar to the effect of
                systemctl(1)'s
                mask command. This is useful to
                boot with certain units removed from the initial boot
                transaction for debugging system startup. May be
                specified more than once.
If the systemd.wants= option is
                specified and followed by a unit name, a start job for
                this unit is added to the initial transaction. This is
                useful to start one ore more additional units at
                boot. May be specified more than once.
If the systemd.debug-shell
                option is specified, the debug shell service
                "debug-shell.service" is pulled into
                the boot transaction. It will spawn a debug shell on
                tty9 during early system startup. Note that the shell
                may also be turned on persistently by enabling it with
                systemctl(1)'s
                enable command.
systemd-debug-generator
                implements the generator
                specification.