fence line vs boundary line

fence line and boundary line both are nouns.

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

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

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

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