Traditionally, in Navy ships, decks
run the length of a hull, flats
are partial "decks" and levels
are superstructure "decks."
IIRC, deck numbering begins with the main deck/weather deck as deck 1 and numbers increase as you approach the keel. IOW, Deck 2 is below Deck 1.
The level above the main deck was Level 01 and numbers increased toward the masthead. On the ship I was stationed on, our bridge was on Level 05.
Which often makes me ponder, what if our a priori Bridge = Deck 1