The PetSafe stubborn dog in-ground fence system has the strongest static correction levels than any other dog fence system on the market. It is specifically designed for stubborn dogs weighing over 15 pounds. With the addition of wire and flag kits, the system can be expanded up to 25 acres. The adjustable collar included fits neck sizes ranging from 6-28 inches, making it great for small, medium and large dog breeds. The collar features 5 levels of adjustable correction to teach your dog to retreat back to the safe zone. There's also an additional tone-only with vibration feature that you can use for tailored training.