I suppose transponders takes some of the guesswork out of it for some racing. During a F1's VSC for example in keeping drivers honest. It removes the responsibility of a lap counter and timer always being 100% accurate as well I guess. I have witnessed some confusion by officials at stock car races on small oval tracks, usually when there were a lot of cars on the track at one time. Yellow flags red flags, multiple cars making pit stops, etc.
So are transponders supplied by the track, or club, for each event, or are you required to have your own unit?
Is the radio frequency universal for all tracks?