Occupancy sensors

In short: Match the sensing technology and the mount to the room, then check coverage, mode, and voltage before you buy.

What to look for

Sensing method. PIR watches moving heat and needs a clear line of sight. Ultrasonic hears motion around partitions and stalls. Dual-tech asks both to agree before it turns lights on, which cuts false triggers.
Mount. A wall-switch sensor suits a small private office with a view from the door. A ceiling sensor suits large rooms, open offices, and restrooms with stalls. Aisles and warehouses want a long-range corner or high mount.
Coverage map. Walking coverage (major motion) is larger than typing coverage (minor motion). The minor-motion pattern has to reach the desk, not stop at the doorway.
Mode. Occupancy turns lights on and off. Vacancy only turns them off, so someone has to switch them on. Buy the mode the room actually needs.
Voltage. A line-voltage sensor switches the power itself. A low-voltage sensor sends a small signal to a power pack or relay. Those are not interchangeable.
Adjustments you can reach after install: time delay and sensitivity. Offices often want a longer delay so lights do not drop on someone sitting still.

A PIR sensor in a restroom stall or behind partitions. It cannot see around the wall.
Aiming the sensor out the door, or at an HVAC supply, so the hallway or the airflow holds the lights on.
A timeout so short that the lights drop while people are still in the room.
Vacancy mode when the users expected the lights to come on by themselves.
A line-voltage sensor on low-voltage wiring, or the reverse.
Buying on coverage diameter alone and skipping the minor-motion pattern.

General buying advice only. This guide does not speak for any manufacturer, and it names no brands or part numbers. The shop is separate.