Two small devices on the same secured door, answering two completely different questions — and a system that can't tell a legitimate exit from a break-in without both of them.
Walk up to almost any electrified access-controlled door and you'll find two low-voltage devices that get lumped together as "door hardware" and treated as roughly interchangeable. They aren't. A door position switch (DPS) — usually a magnetic contact or reed switch split across the frame and the door leaf — reports one thing and one thing only: is this door physically open, or is it physically closed. A request-to-exit (REX) device— usually a ceiling-mounted PIR motion sensor or a push-button on the secure interior side — has a completely different job: telling the access control system that someone inside intends to leave, so the lock can release and the alarm logic can stand down, without ever asking that person for a credential. Confuse the two, or assume either one is optional because "the other one basically covers it," and the door either can't distinguish a legitimate exit from a forced entry, or loses the ability to know its own physical state at all.