sustain vs document

sustain is not a noun while document is a noun.

Word	Noun	Adjective	Verb	Adverb
sustain	No	No	Yes	No
document	Yes	No	Yes	No

As verbs, document is a hyponym of sustain; that is, document is a word with a more specific, narrower meaning than sustain:

Other hyponyms of sustain include back, back up, vouch, verify, demonstrate, establish, prove, shew, show, validate.

sustain (noun)	document (noun)
	anything serving as a representation of a person's thinking by means of symbolic marks
	writing that provides information (especially information of an official nature)
	(computer science) a computer file that contains text (and possibly formatting instructions) using seven-bit ASCII characters
	a written account of ownership or obligation

sustain (verb)	document (verb)
undergo (as of injuries and illnesses)	support or supply with references
establish or strengthen as with new evidence or facts	record in detail
admit as valid
supply with necessities and support
provide with nourishment
be the physical support of; carry the weight of
lengthen or extend in duration or space