In dead-deckoning mode, and to verify the GPS signal, the nav uses the inbuilt gyro compass, and the external wheel pulses to update the current position.

Wheel pulses however do not indicate if the vehicle is moving forwards or backwards, they only indicate speed.

So the nav requires the reverse gear signal to determine if the car is moving forwards or backwards.

Vital for correct position calculation under condiitons of bad GPS signals, eg: in car parking buildings, tunnels, densly built up areas, or even days of heavy rain and dense fog when the GPS signal gets weak (like I had last Friday - my GPS strugggled and calculated huge position and speed errors, at times I was doing 250kmph - because my GPS had no dead reckoning capability, therefore could only use GPS signals, which sometimes have large errors)