•  In-Only - When the pet comes in, he cannot go back out again. One possible use would be a case where you want the pet in at night. You set the pet door for "in only" in the evening.
•  Out-Only - When the pet goes out, he cannot come back in again. A possible use for this setting would be a case where you want the pet out during the day. You'd set the flap on "out only" in the morning.
•  Locked - This setting allows no entry in either direction.
•  Open - This setting allows entry in both directions.

Sometimes, the 4-way locking mechanism is incorporated in an electronic pet door. In these cases, the 4-way lock may over-ride the electronics. For example, your cat wears a collar key so only he may come in your pet door. But, if your pet door is also set for 'out only' no animal, including your cat, may come in.

Cat flaps with 4-way locks always have rigid flaps which are necessary for the mechanism to work.

To the right are two examples of pet doors that incorporate a 4-way lock. There are several others.