A few units are treated specially by systemd:

basic.target, bluetooth.target, ctrl-alt-del.target, cryptsetup.target, dbus.service, dbus.socket, default.target, display-manager.service, emergency.target, exit.target, final.target, getty.target, graphical.target, halt.target, hibernate.target, http-daemon.target, hybrid-sleep.target, kbrequest.target, kexec.target, local-fs.target, local-fs-pre.target, mail-transfer-agent.target, multi-user.target, network.target, nss-lookup.target, nss-user-lookup.target, poweroff.target, printer.target, reboot.target, remote-fs.target, remote-fs-pre.target, rescue.target, rpcbind.target, runlevel2.target, runlevel3.target, runlevel4.target, runlevel5.target, shutdown.target, sigpwr.target, sleep.target,       smartcard.target, sockets.target, sound.target, suspend.target, swap.target, sysinit.target, syslog.socket, syslog.target, system-update.target, time-sync.target, umount.target

(c) man systemd.special