I have two, one from the hall to the garden room, placed backwards, to keep unprogrammed cats inside, and the second in the garden room leading into the garden, placed the usual way round, to keep strange cats out.
If I have a new cat , he cannot go into the garden room, and therefore not go out, while the other cats can. When the new cat has settled in and is allowed out, he will be programmed into the cat flaps, too.
I am quite happy with my cat flaps, though some cats will manage to get in, anyway, especially if they are close friends of mine. If they follow close enough behind, the resident cat's tail will keep the flap open for his friends to come in.
My red tom Jiskefet used to do that on purpose (we had a magnetic flap with a collar key in those days).
He could always get in if he lost his collar by closely following one of the others.