According to the AIM, in class B and C airspace separation of VFR traffic is "suspended during CENRAP operations":
Separation of VFR aircraft will be suspended during CENRAP operations. Traffic advisories and sequencing to the primary airport will be provided on a workload permitting basis. The pilot will be advised when center radar presentation (CENRAP) is in use.
What is CENRAP, how are pilots informed that it's in use (i.e. what is the radio phraseology?) and what action is expected from pilots in that case?