The primary bus name of a service is added to the service's unit file, e.g:
BusName=org.kde.KWin
This is used by systemd to detect the point where a service is considered to be "ready".
systemd will wait until the service has registered on the bus using this name before it
returns the state active for it.
All bus names (incl. the primary) go to a dbus session.d/ file, where each file represents
a bus name registered by this service. The service is activated via D-Bus, e.g.:
SystemdService=org.kde.KWin