The B, the commonest entry, stands for Brig, a two masted squaresail rig. The SL stands for SLoop, a one masted fore and aft rig. The SN I guess stands for SNow. Statistically all the largest ships have no entry and are therefore ship rigged. SL only appears with the smallest vessels.
B tends to be associated with middling vessels, although there is more variance here.
The W seems to wander around somewhere near the middle of the column. I think this stands for Low Waist, a ship with a high forecastle and quarter deck as opposed to a flush deck vessel. The norm is, where there is no information to the contrary, the vessel has two decks and three masts in standard square rigged layout. The most important assuption is that all vessels were of two decks unless otherwise stated. In the Port column, the presence of a line does not signify the absence of data, it means 'ditto', that the entry is to be read as being the same as the entry in the line above.