I don't think it is nessesarily an NEC thing. 210.70 touches on it but does not define what a "habitable room" is. Nor does it mention closets. I guess if you can walk inside it, it would be habitable. But maybe in other building codes for rooms of certain size etc, to have so many lumens of light. Most building codes also have mandated available daylight too, for all but closets.
As for storage areas, 210.70 (A) 2+3, its kinda vague and at the same time specific to certain places, but does not specifially say "closets" at all.
[This message has been edited by e57 (edited 05-02-2005).]