The Plexidor Pet Doors and Plexidor Wall Dog Doors have rigid flaps that are sealed to the frame with this fuzzy weatherstripping.
The weather strips slide into a channel around the edge of the flap, and are glued in with silicone. If the weather strips get damaged, they can be replaced without having to replace the entire flap. It is a bit of a chore to scrape the old silicone off of the flap and reinstall the new piece, so if you want to make it easier on yourself or if your flaps are scratched up and worn looking, you might consider replacing the entire flap, as the new ones come with the weather strips already installed.
If your Plexidor dog door has one flap open all the way to the inside or outside, you need a spring kit.