There may be innumerable (ironic) ways for someone else to explain what it means, but we are here to understand what you mean. The only logical way to achieve that is for you to tell us.

1. If you have a pet sum type definition, let it be known. But traditionally, a sum type is better known as a discriminated or tagged union. So far, this is what we understand Pascal offers: A union type that discriminates its subtypes by an enumerated value.

2. The tag that discriminates the type within the union (or whatever your explanation above ends ups calling it) is, in implementation, generated by a process that assigns a number to each entity; something also true of Rust. This is undeniable, as proven by the use of Ord and mem::discriminant. If you have another name for that numberer, if not an enumerator, let it be known.