The additional SmartKey electronic key is used for other domestic pets with the SmartDoor ™ electronic door. The SmartDoor electronic door ™ detects the radio frequency signal from the SmartKey electronic key that your pet has hung on its collar. The key unlocks the door and the pet can go through it. As soon as the door stops registering the signal from the SmartKey, the damper closes automatically. The door can detect up to 5 programmed SmartKeys.