systemd-rc-local-generator — Compatibility generator for starting /etc/rc.local during boot
/usr/lib/systemd/system-generators/systemd-rc-local-generator
systemd-rc-local-generator is a generator that checks whether
    /etc/rc.local exists and is executable, and if it is pulls the
    rc-local.service unit into the boot process. This unit is responsible for running
    this script during late boot. Note that the script will be run with slightly different semantics than the
    original System V version, which was run "last" in the boot process, which is a concept that does not
    translate to systemd. The script is run after network.target, but in parallel with
    most other regular system services.
Support for /etc/rc.local is provided for compatibility with specific System
    V systems only. However, it is strongly recommended to avoid making use of this script today, and instead
    provide proper unit files with appropriate dependencies for any scripts to run during the boot process.
    Note that the path to the script is set a compile time and varies between distributions.
systemd-rc-local-generator implements
    systemd.generator(7).