A sum type is a tagged union [1]. In C, a sum type can be modeled by the combination of an enum and a union. The enum would tell you which field of the union you should use. In Rust, enum's subsume unions and enums of C. This is analogous to a a disjoint union [2] in set theory. Notice that in both cases you have a pair of data. Sum types and disjoint unions are both instances of coproducts in category theory, by the way.
1. https://en.wikipedia.org/wiki/Tagged_union
2. https://en.wikipedia.org/wiki/Disjoint_union