Motion sensors as the name suggest detect motion within the field of view in case of PIR. If there is no motion, there is no detection. For example, it cannot detect a sleeping person as there will be not movement. Occupancy sensors on the other hand detects the continuous presence of a person/persons within the detection zone. As such motion detection sensors are useful for short duration application which involve intermittent (in seconds) motion detection, like in toilets or bathrooms, pathways, hotel or building corridors. Occupancy sensors are more suited for long duration (in hours) like bedrooms, living rooms, office cubicles, hotel rooms, etc.