On naval vessels, a "deck" is a major horizontal division in the ship's structure, particularly in the hull. The superstructure that sits on top of the main deck of a warship is said to be divided up into levels, while deeper in the hull you have still more decks between the main deck and the keel. Put that another way, a "deck" is basically the horizontal version of a "bulkhead." Because we're trekkies, we like to think of "bulkhead" as a nautical term for "wall" but again, on naval vessels, this refers to specific vertical dividers that break up the hull into compartments. So an aircraft carrier gets hid amidships by a torpedo, for example, the impact point might be, say, "Between bulkheads 38 and 39, just below E-deck." If your deck is ten meters high, you can fill that space with anything you want. Catwalks, for example, are not decks, neither are platforms, stairways, landings, bookshelves, etc. Most naval vessels use "decks" as separate floors mainly because they are small and because it's easier to keep the ship airtight if you break up the space into smaller chunks. Submarines do not always have this feature, and starships are large enough that they almost certainly don't. That or we have a fundamental misunderstanding of what a "deck" is on a starship. There seems, for example, to be a LOT of open vertical space on the Enterprise. We keep seeing corridors that open into spaces that have open roofs to the next level up, so that "room" is actually five-story shaft spanned by catwalks from one side to the other. I'd hazard a guess that any particular "deck" has at least four levels, which is probably a feature of how the ship was actually built: if each "deck" has an independent pressure hull, then its internal arrangement can be broken up into different floors and spaces for crew habitation, or you can fill the entire thing with machinery, cargo, water turbines and warp cores. IOW, "Deck 13" could be as much as 38 stories down from the bridge.