Service monitoring rules add the corresponding service during discovery phase of the host, when these items are found.
The enforced rule adds services statically, without a discovery.
With enforced services you can e.g. expect windows services or processes to be existent or not existent, even when these do not exist on the server.
E.g. you can check if a specific antivirus service is present or you can check the other way round, that teamviewer processes are not running.