AS far as the rentals that they hand out at the tracks, the PC has no idea who has which transponder. The PC just reads when each transpoder passes by the loop. When the race director feeds the names into the PC, the computer program used asigns a transponder # to each name in each heat. The PC doenst know which cars has which transponder, it just assumes that car #3 has transponder #3.
About the personal ones that you can buy, those have a unique ID number. When the race director fills your name in the PC, he has to tell the program that your name should always have that ID number.
Hope it makes sence