captain vs flag captain

captain and flag captain both are nouns.

Word	Noun	Adjective	Verb	Adverb
captain	Yes	No	Yes	No
flag captain	Yes	No	No	No

As nouns, flag captain is a hyponym of captain; that is, flag captain is a word with a more specific, narrower meaning than captain:

captain (noun)	flag captain (noun)
a dining-room attendant who is in charge of the waiters and the seating of customers	the captain of a flagship
the pilot in charge of an airship
the naval officer in command of a military ship
an officer who is licensed to command a merchant ship
a policeman in charge of a precinct
an officer holding a rank below a major but above a lieutenant
the leader of a group of people

captain (verb)	flag captain (verb)
be the captain of a sports team