The Extra Remote for the Houndware HW900 Waterproof Outdoor Remote Dog Training Collar For Stubborn Dog serves as the transmitter unit for this training system. If you've misplaced or experienced damage to your current transmitter unit, this remote provides a convenient replacement option. By acquiring this transmitter, you can seamlessly continue utilizing the Houndware HW900 Waterproof Outdoor Remote Dog Training Collar For Stubborn Dog.