IOU vs note of hand

IOU and note of hand both are nouns.

Word	Noun	Adjective	Verb	Adverb
IOU	Yes	No	No	No
note of hand	Yes	No	No	No

As nouns, note of hand is a hypernym of IOU; that is, note of hand is a word with a broader meaning than IOU:

IOU: an informal debt instrument; representing `I owe you'
note of hand: a promise to pay a specified amount on demand or at a certain time

Other hypernyms of IOU include note, promissory note.

IOU (noun)	note of hand (noun)
an informal debt instrument; representing `I owe you'	a promise to pay a specified amount on demand or at a certain time