I've never installed one, but rule 30-204 states:
(1)Every luminaire installed in a clothes closet shall be located on the ceiling or on the front wall above the door of the closet .......etc
(2)Lampholders and luminaires or the pendant or suspended type, and lampholders and luminaires of the bare lamp type shall not be installed in clothes closets.
I believe they are usually some type of sealed/covered fixture to minimize the fire hazard. (with so many combustable objects crammed in a closet.)
