fence line vs borderline

fence line and borderline both are nouns.

Word	Noun	Adjective	Verb	Adverb
fence line	Yes	No	No	No
borderline	Yes	Yes	No	No

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

Other hypernyms of fence line include border, boundary line, delimitation, mete.

fence line (noun)	borderline (noun)
a boundary line created by a fence	a line that indicates a boundary

fence line (adjective)	borderline (adjective)
	of questionable or minimal quality