underside vs sole

underside and sole both are nouns.

Word	Noun	Adjective	Verb	Adverb
underside	Yes	No	No	No
sole	Yes	Yes	Yes	No

As nouns, sole is a hyponym of underside; that is, sole is a word with a more specific, narrower meaning than underside:

Other hyponyms of underside include base, bilge, heel, underbelly.

underside (noun)	sole (noun)
the lower side of anything	right-eyed flatfish; many are valued as food; most common in warm seas especially European
	the underside of footwear or a golf club
	the underside of the foot
	lean flesh of any of several flatfish

underside (adjective)	sole (adjective)
	being the only one; single and isolated from others
	not divided or shared with others

underside (verb)	sole (verb)
	put a new sole on