hiya - ratings are for both frame and glass strength - for residential frame they usually weld a blade at 90 degrees to teh ddor on the mullions to give it strength for wind. For commercial frames, the frames are already 100 * 50mm aluminium versus 50mm by 30mm (or thereabouts) for res frame. Thus commercial framing might end up with neat box section instead of blades coming into the room for the door size. If the door is only 2100 high, then std capral commercial suite would be at least N3 and possibly N4 with 6.38 laminate glass
By and large, companies stick the rating in the computer, and the computer spits out the design and price. As size of opening and glass are relevant, there is no std easy answer, unless you call , "it depends" an answer?