CGroup system.slice/docker.service – what is that?

● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; disabled; vendor preset: enabled)
   Active: active (running) since Wed 2019-04-10 19:54:37 CEST; 10min ago
     Docs: https://docs.docker.com
 Main PID: 18433 (dockerd)
    Tasks: 95
   CGroup: /system.slice/docker.service
           ├─18433 /usr/bin/dockerd -H fd://
           ├─18448 docker-containerd --config /var/run/docker/containerd/containerd.toml --log-level info
           ├─20373 docker-containerd-shim -namespace moby -workdir /var/lib/docker/containerd/daemon/io.containerd.runtime.v1.linux/m
           └─20398 sleep 120

Hello,

I wonder what does it mean that above processes are in CGroup /system.slice/docker.service?

Source: StackOverflow