ADS-B: Automatic Dependent Surveillance - Broadcast can be and is used by ATC for anything that secondary surveillance radar (SSR) is used for. But that is the issue both ADS-B and SSR are 'cooperative' surveillance, the aircraft is broadcasting its position or replying to the SSR interrogation with a reply. If the controller only has ADS-B and SSR then the surveillance will not show non-cooperative aircraft. So once the aircraft is in airspace that is not mandatory SSR/ADS-B, the airspace is shared with aircraft legally flying without ADS-B/SSR and in some airspace not even required to have radio contact with ATC. Therefore, the controller can no longer guarantee separation from all aircraft only from other cooperative aircraft. This becomes important for example when aircraft follow a TCAS RA in non-mandatory airspace and there is no guarantee that the RA is safe as TCAS (which uses a version of SSR) will not see non-cooperative aircraft.