The Extra Collar for the serves as the receiver unit for this training system. If your current collar unit from the Houndware HW900 Waterproof Outdoor Remote Dog Training Collar has been misplaced or damaged, acquiring this replacement collar offers a straightforward and convenient solution. With this new collar, you can effortlessly resume your training routines, ensuring that your training regimen for your stubborn dog remains consistent and effective. This replacement option ensures that your Houndware HW900 system continues to function optimally, maintaining the quality and efficacy of your dog training efforts.