Exactly. I like to pick my own also. It is confusing about the Categories. And they are different sometimes between the ships.
The number refers to the type of cabin.
The letter refers to the deck.
Example: 6A refers to an Outside Cabin on Riviera Deck. 6D refers to the same Outside Cabin except on a higher deck, Empress Deck.
1A is always upper/lower ie. Bunkbeds Inside.
4A is Inside Twin/King on Riviera, 4D would be the same except on Empress Deck.
The Cabins are identical except for the deck.
Hope this helps explain the crazy way they do this