split-rail fence vs fencing

split-rail fence and fencing both are nouns.

Word	Noun	Adjective	Verb	Adverb
split-rail fence	Yes	No	No	No
fencing	Yes	No	No	No

As nouns, fencing is a hypernym of split-rail fence; that is, fencing is a word with a broader meaning than split-rail fence:

split-rail fence: a fence (usually made of split logs laid across each other at an angle)
fencing: a barrier that serves to enclose an area

Other hypernyms of split-rail fence include fence.

split-rail fence (noun)	fencing (noun)
a fence (usually made of split logs laid across each other at an angle)	the art or sport of fighting with swords (especially the use of foils or epees or sabres to score points under a set of rules)
	a barrier that serves to enclose an area
	material for building fences