The Sensor is used to automatically activate the reset function on the AquaTrip if it has been tripped. For example - if fitted to a public toilet block, and vandals have broken a tap and water is flowing non stop. The AquaTrip will shut off the water, but if some one enters the facility, the Sensor will detect their presence and cause the AquaTrip to turn the water back on for as long they are in the facility. After they leave the water ill be shut off again. On the AT201, you must use the Part # PBC-AT to connect the Sensor to the Control Unit.