butter-bean plant vs shell bean

butter-bean plant and shell bean both are nouns.

Word	Noun	Adjective	Verb	Adverb
butter-bean plant	Yes	No	No	No
shell bean	Yes	No	No	No

As nouns, shell bean is a hypernym of butter-bean plant; that is, shell bean is a word with a broader meaning than butter-bean plant:

butter-bean plant: bush bean plant cultivated especially in southern United States having small flat edible seeds
shell bean: a bean plant grown primarily for its edible seed rather than its pod

Other hypernyms of butter-bean plant include shell bean plant.

butter-bean plant (noun)	shell bean (noun)
bush bean plant cultivated especially in southern United States having small flat edible seeds	unripe beans removed from the pod before cooking
	a bean plant grown primarily for its edible seed rather than its pod