You have raised an interesting question. For the most part, any loft accessed via a ship's ladder cannot be used as a sleeping area, and in some cases only for storage code wise. So most of them are based on that requirement. However, on some of our larger plans, we still call it a loft even thought it has a conventional stair because there are three bedrooms, and an open loft usage is more likely than an additional sleeping space.
Bottom line, if it has a stair you can call it a bedroom.
Hope this clears this up, if not let me know