Where the ship is a two decker, the right side entries sometimes appear on the left side, notably where a B is the only entry. It should actually be on the right. My suggestion is that the rightmost entries are about rigging, or number of masts, or ship types, depending on how you think about them. The entries are B, SL, SN and Bark. I believe there is an unstated assumption that all vessels are ship rigged unless otherwise stated.
Ship rig is three masts with squaresails.
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.