PDC... modules should be used when "simple" disconnection of PDC unit of rear park-sensors (directly using of output from WHx/MPx modules) makes a troubles - like eg. signalling the device malfunction in car.

PDC-OFF
PDC-OFF module was designed for disconnection the signal circuit (buzzer or input) in factory cabling-system of main unit of rear park sensors (PDC) when trailer is connected.
PDC-SWITCH-OFF
PDC-SWITCH-OFF module was designed for automatically simulation of pushing the factory button deactivates rear parking sensors of car when trailer is connected.