Hue Motion Sensor Entities

When adding a Hue motion sensor through ZHA, there are 5 entities added.
Temperature, Power and Illuminance are self explanatory.
I wish Occupancy was consistent with other motion sensors, but oh well - it can be renamed.
And then the fifth one is just called on_off with a seemingly permanent status of Closed. I can’t figure out what this does - nothing has ever changed the status. Is there a simple explanation that I’ve overlooked?

Thanks!

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