Simple way to think about it is plus. Sum plus sum increases, union plus union does nothing.

1. A union type is like int | double. It means the value is one of a set of possible types. And it's a true set: `int | int | double` is indistinguishable from `int | double`.

2. A sum type is a new type built from a list of other types, which assigns a 'tag' to each possible list element, like a Rust enum. The tags are not themselves types: they are like a struct field name. It's quite possible to have a sum type with N tags, each wrapping `int`.

So with this understanding Rust has sum types but not union types, while TypeScript has union types but not sum types.