That probably means top bee space. In a BS National the frame tops are flush with the rims of the box and there is more space underneath the frames. Crownboards have wooden strips around the edge and these go on the lower surface of the board so that there is a bee space above the frame tops.

Place a top bee space feeder on a bottom space box of bees (as I did last year) and there is an unpleasant crunching sound!

Smiths are the second most popular hive type amongst locals (some, including our biggest bee farmer locally, have Langstroths). They all have a gap between the frame tops and the rim of the box, and take flush crownboards (with the edging on the top side).