Those are sections of the Third Class accommodation. In First and Second class the staterooms were given a deck letter and number, for example C 16 would be on the on the port side of C Deck. In Third Class the cabins were simply numbered from 1 to 221 or so, but the sections of the ship were given letter designations. Section "A" was the 3rd Class Open Space on D Deck. Section "B" was immediately beneath A on E Deck. C was below B and D was below C on G Deck. Then came section E on F Deck; section F was on G Deck, and then G on E Deck.
It worked something like this:
Aft Forward
D Deck: O x x x x x x A
E Deck: Q M K x x x x B
F Deck: R x x J H G E C
G Deck: S N x x x x F D
Ignore the x's, I had to put them in so the table would align properly. A, J and H were public spaces, not staterooms, and there was no I. I'm not sure about L and P, they may have been reserved for 2nd class spaces that could be used as 3rd class if needed.
Does this help? I know it's a little confusing but that's how they designated Third Class spaces on Titanic.
