The TCM is recognized by the vehicle, and can then advise the vehicle when a trailer is connected to the external trailer connection.

The TCM monitors the external socket and if it detects a trailer, then advises the vehicle, which disables the rear PDC, and activates another mode in the DSC algorithms that is called trailer stability control. I presume it also tells the LCM, because bulb monitoring then extends to trailer lights. If there is a rear camera (some models) it can change the view to facilitate connecting the trailer.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue