The PetSafe Wireless Fence Extra Receiver allows you to add additional dogs to the PetSafe Wireless Containment System. The number of dogs the system contains is only limited by the number of receiver collars you own. One wireless transmitter unit covers an adjustable circular area of 180 ft. in diameter, or around ½ acre. Fully waterproof with 5 adjustable levels of static correction and a tone-only setting. Operates on the included user-replaceable RFA-67D battery and the red adjustable collar fits neck sizes 6-28 in.