sustainment vs upkeep

sustainment and upkeep both are nouns.

Word	Noun	Adjective	Verb	Adverb
sustainment	Yes	No	No	No
upkeep	Yes	No	No	No

As nouns, sustainment and upkeep are synonyms defined as:

sustainment and upkeep: the act of sustaining life by food or providing a means of subsistence

Other synonyms of sustainment include maintenance, sustenance, sustentation.

sustainment (noun)	upkeep (noun)
the act of sustaining life by food or providing a means of subsistence	activity involved in maintaining something in good working order
	the act of sustaining life by food or providing a means of subsistence