The door comes with an ultrasonic transmitter for the dog’s collar, and opens/closes based on “hearing” that sound approaching the door from either side.
The reason why they DIDN’T use a simple RFID tag or pressure mat is because the ultrasonic microphones in the door can “hear” the doppler shift in the sound of the dog as he/she walks towards the door, ans will only open it if the dog is moving towards the door. If he/she falls asleep inside or outside right in front of the door, it will remain closed.
There’s a tip above about how to save $$$ on replacement batteries for the collar transmitter, and a few YouTube videos on the subject if you search there…