CCDS data
At the Prestwick Centre the Code Callsign data (CCS) is derived the Code Callsign Database/Distribution System (CCDS) at Swanwick. The CCS is unique and is tied to the mode 3/A code that the aircraft squawks. This is the same for standard SSR and Mode S secondary radar. Most airports also connect to CCDS so they also have the same information displayed for aircraft as the centres.
CCDS obtains the callsigns from Civil flight plan system NAS and Military equivalent system EDDUS.