understructure vs raft

understructure and raft both are nouns.

Word	Noun	Adjective	Verb	Adverb
understructure	Yes	No	No	No
raft	Yes	No	Yes	No

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

understructure: lowest support of a structure
raft: a foundation (usually on soft ground) consisting of an extended layer of reinforced concrete

Other hyponyms of understructure include bed, mat, raft foundation.

understructure (noun)	raft (noun)
lowest support of a structure	a flat float (usually made of logs or planks) that can be used for transport or as a platform for swimmers
	a foundation (usually on soft ground) consisting of an extended layer of reinforced concrete
	(often followed by `of') a large number or amount or extent