The dedicated wiring loom will detect the additional resistance from the trailer bulbs,
the cars ECU will then disable the reverse sensors and the cars rear fog lights and
possibly even the reversing lights.
It usually requires a trip to a dealer unless the towbar fitter has the equipment to
program it.
There have been issues with some dealers not knowing what they were doing though
and I remember one member having 3 trips to the dealer before it worked properly.
If towing a caravan and you want to charge the leisure battery and run the fridge check
thats included, often its extra.