That's EASY, it is its own genre of photography by far... given its global economic impact and breadth of use.
That said I think the sub-categories of "stock" are those, nature, wedding, portrait, candid, erotic, macro, even artistic or interpretive. Hmm, definition of stock photography... art that is produced to sell after it is produced as opposed to commissioned, in any genre. It's a catch-22, isn't it?
Good point, man.