union card vs membership card

union card and membership card both are nouns.

Word	Noun	Adjective	Verb	Adverb
union card	Yes	No	No	No
membership card	Yes	No	No	No

As nouns, membership card is a hypernym of union card; that is, membership card is a word with a broader meaning than union card:

union card (noun)	membership card (noun)
a card certifying membership in a labor union	a card certifying membership in an organization