sole vs underside

sole and underside both are nouns.

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

As nouns, underside is a hypernym of sole; that is, underside is a word with a broader meaning than sole:

Other hypernyms of sole include bottom, undersurface.

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

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

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