sum vs subtotal

sum and subtotal both are nouns.

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

As nouns, subtotal is a hyponym of sum; that is, subtotal is a word with a more specific, narrower meaning than sum:

Other hyponyms of sum include grand total.

sum (noun)	subtotal (noun)
the whole amount	the sum of part of a group of numbers
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

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