lower deck vs deck

lower deck and deck both are nouns.

Word	Noun	Adjective	Verb	Adverb
lower deck	Yes	No	No	No
deck	Yes	No	Yes	No

As nouns, deck is a hypernym of lower deck; that is, deck is a word with a broader meaning than lower deck:

lower deck (noun)	deck (noun)
the deck below the main deck	any of various platforms built into a vessel
	street name for a packet of illegal drugs
	a porch that resembles the deck on a ship
	a pack of 52 playing cards