A vs B = an A type room would be more midship, while a B type is more forward or aft of midship. And Cs are even more forward/aft or on a lower deck than the A/B types (same category)It's mostly differences based on the location of the stateroom on the ship.
For example, I believe the difference between 6A and 5A is that 6A is on a higher deck than a 5A.
The difference in a 6A vs a 6B might be in the style of the veranda (solid railing vs plexiglass).
I believe the Navigator Verandahs on the Dream class only have the solid wall up to about shin-high and plexiglass above to the rail.On the Fantasy and Dream a 6A is on deck 8 and has a navigators veranda - solid from floor to railing. The cat 5s have veranda with thick glass of plexiglass from the floor to the railing giving a more open feeling.
Deck Category
10. 5A
9. 5A
8. 5B
7. 5C
6. 5D