Originally Posted by
MarkerInbound
Mode A and C transponders says "Hi, I'm 1234." The ATC system has been told 1234 is FastJet 100 and so the controller sees FastJet 100 with the data block. With a Mode S Transponder the flight enters FastJet 100 in FMS as part of the setup and the transponder says "Hi, I'm FastJet 100."
In the UK, it doesn't matter what transponder the aircraft carries, individual radar displays can 'convert' even a Mode A label to the aircraft callsign if it's entered manually and this will automatically be displayed on all of the radar unit's displays.