I think Toadpool is referring to the UK. In UK controlled airspace you can usually work out who is providing a service to an aircraft by it's position. Outside controlled airspace it's not so easy as an aircraft may be receiving a service from one of several local units, from one of several military units or not receiving a service at all. To cope with this the UK allocates blocks of Mode A codes to units which allows other controllers to identify who is controlling traffic which may affect their own aircraft. As Toadpool says this easy useful scheme won't work with Mode S and NATS and the Military are currently figuring out what to do instead.