on the aerodrome surface when under tow, or parked and prior to selecting OFF or STDBY
I think it's to do with the A-SMGCS - Advanced Surface Movement Guidance and Control System - a development of old ground movement radars. I can't remember all the details but the A-SMGCS tracks aircraft (and sometimes vehicles) with multilateration using mode S. Setting 2000 is something to do with having a target showing up but not tagged with the callsign of another flight.