The Volvo system uses specific hardware and software and has the following features:
- Bulb failure warning system extended to trailer (quite comprehensive on later models)
- Fog light on car disabled when trailer attached (programmable)
- Parking sensors disabled when trailer attached
Some aftermarket kits can also do the above, but need software.
And most importantly....
- Trailer stability assist added to the DTSC program
https://www.youtube.com/watch?v=VIMxVzUnBSA
Only the Volvo system does the above.