The init systems - being the last part of the boot sequence - are responsible for setting up the user space programs.
SysV (System 5) - is a init system used before systemd.
SysV had runlevels - 0 - 6. These run-levels specify what should be course of actions that the SysV should do.
runlevel 5 sets up the Graphical system runlevel 3 sets up the multi-user system runlevel 0 is poweroff
The command run-level gives what the level the currently set up
The advantage of using systemd is, systemd reduces the boot time, leveraging parallelism, and runlevels are replaced by systemd targets
To identify which init system is being used
ls -l /sbin/initThe command systemd get-default gives you the default systemd target.
The default systemd target is a symlink to targets present in /lib/systemd/system/*.target
the default target can be changed to systemd set-default
it is possible to set the default systemd target to poweroff. Doing so will shut down your system as you boot it. Recovery is to get into rescue mode using external media