These are sensors that can be added to the Leak Detector Pro to increase the number of zones to be monitored (max 4 zones).  The Leak Detector Pro includes 1 sensor as standard.  Up to three additional sensors can be added to the system for 4 zone coverage.  Sensors do not include wire for field wiring.  Low voltage 2-conductor wire between 22 AWG and 14 AWG is recommended, with a maximum length of 100 feet.