"B" and "A" in the T&S refer to a new bid or new ask (not an actual sale) that improves on the last sale. e.g. 646/648 is the bid/ask, a sale at 646, a new bid at 647 (making the market 647/648) will read 646, 647B, ... in the T&S. 1/ it gives a more representative view on the current market...