[gf000]

I think your "sealed" is misleading here, as that is used for sum types in similar languages (java).

As the language designer notes in the comments, these are named unions, as opposed to anonymous ones, but they are also working on the latter.

"Sealed" is probably not the correct word to use here, as it would be sealed in both case (it doesn't really make sense to "add" a type to the A | B union). The difference is that you have to add a definition and name it.