From man systemd:

DESCRIPTION
       systemd is a system and service manager for Linux operating systems. When run as first process on boot
       (as PID 1), it acts as init system that brings up and maintains userspace services. Separate instances
       are started for logged-in users to start their services.

       systemd is usually not invoked directly by the user, but is installed as the /sbin/init symlink and
       started during early boot. The user manager instances are started automatically through the
       user@.service(5) service.

       For compatibility with SysV, if the binary is called as init and is not the first process on the
       machine (PID is not 1), it will execute telinit and pass all command line arguments unmodified. That
       means init and telinit are mostly equivalent when invoked from normal login sessions. See telinit(8)
       for more information.

       When run as a system instance, systemd interprets the configuration file system.conf and the files in
       system.conf.d directories; when run as a user instance, systemd interprets the configuration file
       user.conf and the files in user.conf.d directories. See systemd-system.conf(5) for more information.