This allows them to be managed by systemd --user on systems that use
it, meaning they can be manipulated by systemctl --user, placed into a
slice for resource management purposes, configured with drop-in
configuration in {/etc,/usr/lib}/systemd/user/goa-daemon.service.d/*.conf
and so on.
On systems that do not launch dbus-daemon as a systemd --user service,
including systems that do not use systemd at all, the SystemdService
field is ignored and the dbus-daemon will continue to use the
traditional activation mechanism. In this case the GOA services will
continue to be child processes of the dbus-daemon.
See also gnome-session#74 (closed).
/cc @bberg