The former is dead, the latter is the new name. See:
http://cgit.freedesktop.org/systemd/systemd/commit/?id=
b2c9cbafb8b1a2742978e1faf37fbb3cf4fb5d44
https://fedoraproject.org/wiki/Features/DisplayManagerRework
[Unit]
Description=Show Plymouth Halt Screen
-After=getty@tty1.service prefdm.service plymouth-start.service
+After=getty@tty1.service display-manager.service plymouth-start.service
Before=halt.service
DefaultDependencies=no
ConditionKernelCommandLine=!plymouth.enable=0
[Unit]
Description=Show Plymouth Reboot with kexec Screen
-After=getty@tty1.service prefdm.service plymouth-start.service
+After=getty@tty1.service display-manager.service plymouth-start.service
Before=kexec.service
DefaultDependencies=no
ConditionKernelCommandLine=!plymouth.enable=0
[Unit]
Description=Show Plymouth Power Off Screen
-After=getty@tty1.service prefdm.service plymouth-start.service
+After=getty@tty1.service display-manager.service plymouth-start.service
Before=poweroff.service
DefaultDependencies=no
ConditionKernelCommandLine=!plymouth.enable=0
[Unit]
Description=Show Plymouth Reboot Screen
-After=getty@tty1.service prefdm.service plymouth-start.service
+After=getty@tty1.service display-manager.service plymouth-start.service
Before=reboot.service
DefaultDependencies=no
ConditionKernelCommandLine=!plymouth.enable=0