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.