This practice has been in use for many years. It is usually used when IFR traffic is departing from or inbound to an airfield outside controlled airspace and is intended to keep cockpit workload down by not requiring a change to transponder code and also for ATC to identify an aircraft departing from an airfield outdside controlled airspace earlier. For inbounds it allows terminal control to see when a flight has landed. At Farnborough post 'Londoon LARS', we had code/callsign conversion activated in our SSR PAC (Plot Assigner-Combiner) so we got the callsign displayed on our screens instead of just the 4 digit code.
Additionally in the UK at least, the IFR flight plan is automatically activated when the SSR code for a particular flight is detected on radar.
Last edited by chevvron; 5th Oct 2016 at 12:03.