If "multiples" is to "product", "_____" is to "sum" I know this might be a really simple question for those fluent in English, but I can't find the term that describes numbers that make up a sum. 
The numbers of a certain product are called "multiples" of that "product". 
Then what are the numbers of a certain sum called?

To approach the question from another direction: A "multiple" of 7 is a number that is the result of multiplying 7 with something else.
If your try to generalize that to sums, you get something like: A "__" of 7 is a number that is the result of adding 7 to something. But that's everything -- at least as long as we allow negative numbers, and if we don't allow negative numbers, then it just means something that is larger than 7. Neither of these concepts feel useful enough in themselves that it is worth it deciding on a particular noun for it.
