main deck vs deck

main deck and deck both are nouns.

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

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

main deck: the uppermost sheltered deck that runs the entire length of a large vessel
deck: any of various platforms built into a vessel

main deck (noun)	deck (noun)
the uppermost sheltered deck that runs the entire length of a large vessel	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