The man page for scope says:
Unlike service units, scope units manage externally created processes, and do not fork off processes on its own.
On my RHEL server, I can see that gdm belongs to the gdm service. So does its child, gdm-simple-slave. gdm-simple-slave has created gdm-session-worker, but this process belongs to a scope, not the gdm service.
In light of this, what does "externally created process" mean, and what is the difference between a service and a scope?
For those readers who prefer code:
# systemctl status gdm gdm.service - GNOME Display Manager Loaded: loaded (/usr/lib/systemd/system/gdm.service; enabled) Active: active (running) since Sun 2018-03-04 16:27:45 EST; 1 day 3h ago Process: 1330 ExecStartPost=/bin/bash -c TERM=linux /usr/bin/clear > /dev/tty1 (code=exited, status=0/SUCCESS) Main PID: 1307 (gdm) CGroup: /system.slice/gdm.service ├─1307 /usr/sbin/gdm ├─1331 /usr/libexec/gdm-simple-slave --display-i... └─1364 /usr/bin/Xorg :0 -background none -verbos... # ps -ef|grep 1331 root 1331 1307 0 Mar04 ? 00:00:00 /usr/libexec/gdm-simple-slave... root 2032 1331 0 Mar04 ? 00:00:00 gdm-session-worker ... # systemctl status 2032 session-38.scope - Session 38 of user root Loaded: loaded (/run/systemd/system/session-38.scope; static) Drop-In: /run/systemd/system/session-38.scope.d └─90-After-systemd-logind\x2eservice.conf, 90-After-systemd-user-sessions\x2eservice.conf, 90-Description.conf, 90-SendSIGHUP.conf, 90-Slice.conf Active: active (running) since Sun 2018-03-04 21:12:08 EST; 22h ago