systemd-user-sessions.service, systemd-user-sessions — Permit user logins after boot, prohibit user logins at shutdown
systemd-user-sessions.service
/usr/lib/systemd/systemd-user-sessions
systemd-user-sessions.service
                is a service that controls user logins. After basic
                system initialization is complete it removes
                /run/nologin, thus permitting
                logins. Before system shutdown it creates
                /run/nologin, thus prohibiting
                further logins. At the same time it also kills all
                user processes, so that system shutdown may proceed
                without any remaining user processes around.