understock vs buy in

understock and buy in both are verbs.

Word	Noun	Adjective	Verb	Adverb
understock	No	No	Yes	No
buy in	No	No	Yes	No

As verbs, buy in is a hypernym of understock; that is, buy in is a word with a broader meaning than understock:

understock: stock with less than the usual or desirable number or quantity
buy in: amass so as to keep for future use or sale or for a particular occasion or use

Other hypernyms of understock include stock, stock up.

understock (verb)	buy in (verb)
stock with less than the usual or desirable number or quantity	amass so as to keep for future use or sale or for a particular occasion or use