Hi Wok,
This instruction applies to mode S transponders only, (permanently allocated internal code to each aircraft).
The instruction states :-
Heathrow Airport is equipped with Mode S Surface Movement Radar.
Pilots must ensure that:-
1. The aircraft transponder is set to transmit mode S signals from the commencement of pushback.
2. The aircraft transponder is set to transmit mode S signals after landing continuously until the aircraft is fully parked on stand.