An object is an instance of a union class if it is an instance of one of its members.

define-union-class ( class members -- )

Union classes can be introspected:
class-members ( class -- seq )

The set of union classes is a class:

union-class? ( object -- ? )

Unions are used to define behavior shared between a fixed set of classes, as well as to conveniently define predicates.

See also
Mixin classes, Maybe classes, Tuple subclassing