The definition of an outlet. Smoke detectors are usually on a different circuit than the room receptacles, but are still required to be on afci because they are identified as an outlet. So if lights and smokes are said to be outlets, then the same can be said about the lights in a garage which are prohibited to be on the receptacle circuit. I was just using that as an example.
The prohibition of using the garage circuit for lighting has zero to do with AFCIs.
And I usually put smokes on the master BR circuit.