Daffodil currently rejects (as an unimplemented), schemas that use unions of simple types in the restricted way that DFDL supports.

However, some users have requested that this be at least tolerated so that they can put these in their DFDL schemas even if Daffodil doesn't validate against these.

Fact is, we should accept these, and when daffodil's internal validation is used, or checkConstraints() is called, we should validate against these.

See also DFDL-1633 which is about implementing the unionMemberSchema infoset feature.
(same users need that as well as this)