Boot Kernel Command Line
On boot systemd activates (by default), the target unit default.target whose job is to activate services and other units by pulling them in via dependencies.
To override the unit to activate, systemd parses its own kernel command line arguments via the systemd.unit= command line option. This may be used to temporarily boot into a different boot unit. The classical run-levels are replaced as following:
systemd.unit=rescue.target is a special target unit for setting up the base system and a rescue shell (similar to run level 1); systemd.unit=emergency.target, is very similar to passing init=/bin/sh but with the option to boot the full system from there; systemd.unit=multi-user.target for setting up a non-graphical multi-user system; systemd.unit=graphical.target for setting up a graphical login screen.
For details about these special systemd boot units, view the man systemd.special page.