Discovery uses NHS numbers to link separate patient records into a single person/citizen record.
Distinct patient records are linked during inbound data processing using NHS number; all records for the same NHS number are linked together regardless of the originating supplier system (EMIS, TPP, Cerner, Adastra, Vision).
Distinct records are still stored separately, so there's no merging, and if the NHS number on a record changes, then it gets cleanly unlinked and re-linked to any others it should be. The same separation is maintained in the DDS subscriber databases (Compass DB), with each distinct record having its own "patient_id", but they all have a "person_id" which links them together.
All current DDS publishers are PDS-linked so that all NHS numbers sent to DDS have been PDS traced (although some publishers don't explicitly tell us this in their extracts). Therefore Discovery currently trust any NHS number that is sent.
Discovery is starting work on integrating DDS with PDS so that an NHS number can be verified as correct on each patient record.