/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.
systemd-rc-local-generator implements systemd.generator(7).