This is the class for C union types. There are no built-in types. They must be constructed using foreignUnionType.
The object ForeignUnionType is a type, with ancestor classes ForeignType < Type < MutableHashTable < HashTable < Thing.
The source of this document is in /build/reproducible-path/macaulay2-1.25.05+ds/M2/Macaulay2/packages/ForeignFunctions.m2:1460:0.