I too would like to know the answer to that question. I am of the Stewart clan and my "for best" kilt is the ancient Stewart Hunting.
I have noticed that for some reason many, if not all, of the Stewart tartans are listed as "universal" i.e. Royal Stewart, Black Stewart and the Hunting Stewart. I am not sure about the Blue Stewart and the Camel Stewart tartans. Is this because the Stewarts are such a large clan?