county line vs boundary

county line and boundary both are nouns.

Word	Noun	Adjective	Verb	Adverb
county line	Yes	No	No	No
boundary	Yes	No	No	No

As nouns, boundary is a hypernym of county line; that is, boundary is a word with a broader meaning than county line:

Other hypernyms of county line include bound, bounds.

county line (noun)	boundary (noun)
the boundary between two counties	the greatest possible degree of something
	the line or plane indicating the limit or extent of something
	a line determining the limits of an area