subtotal vs sum

subtotal and sum both are nouns.

Word	Noun	Adjective	Verb	Adverb
subtotal	Yes	No	No	No
sum	Yes	No	Yes	No

As nouns, sum is a hypernym of subtotal; that is, sum is a word with a broader meaning than subtotal:

Other hypernyms of subtotal include amount, total.

subtotal (noun)	sum (noun)
the sum of part of a group of numbers	the whole amount
	the choicest or most essential or most vital part of some idea or experience
	a quantity obtained by the addition of a group of numbers
	the final aggregate
	a set containing all and only the members of two or more given sets
	a quantity of money

subtotal (verb)	sum (verb)
	determine the sum of
	be a summary of