We need to keep Daffodil DFDL consistent with IBM's implementation as we know people will want to move schemas back and forth.
IBM had product in the field before the name change from separatorPolicy to separatorSuppressionPolicy, so they are going to support both.
Key: they are treated as the same property even though they have different names and different enum values. This is needed in order for schemas to continue to work that have a mixture of the old and new property names.
This means that you can have say, separatorSuppressionPolicy in some named format that you reference with dfdl:ref, but if you locally define separatorPolicy, it will override the separatorSuppressionPolicy.
Right now these are treated as separate and incompatible in Daffodil. You can't have both in a schema. That's too rigid.