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 or 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.