The same door, the same door contact sensor, and two systems that watch it for two completely different reasons — one running every second of every day, the other switched on and off like a light.
Clients and even some junior technicians routinely lump these together as "the alarm system." They aren't the same system, and they don't even ask the same question. Access control is a continuous, always-active system: at every door, every second, it evaluates whether the person trying to pass is permitted to pass, and grants or denies in real time — occupied or empty, 9am or 3am, it never stops running that evaluation. Intrusion detection is an armed/disarmed-statesystem: it only cares whether the space is currently supposed to be occupied at all. When armed, any motion, any broken window, any door that opens is treated as evidence that someone is present where no one should be. Same building, sometimes the exact same physical devices — but one system is asking "are you allowed through this door," and the other is asking "should anyone be in here right now."