XML Schema: Overview and limitations of coding requirements
Specializations and constraints in XML Schema are implemented using the XSD <xs:redefine> facility. However, this facility does not allow sequence groups to be directly constrained. Thus, to support both specialization and constraints, it might be necessary to refactor content models into named groups that can be redefined. In order to keep the XSD, RELAX NG, and DTD implementations as consistent as possible, the DITA Technical Committee only refactored those content models that were required for OASIS-provided grammars, the strict task body and machinery-industry task. The other DITA content models distributed by OASIS have not been refactored.
You MAY modify OASIS-provided XSD modules to refactor content models if required by your constraint. You SHOULD notify the DITA Technical Committee (TC) of your constraint requirements, so the TC can consider adding the required refactoring to the OASIS-provided XSDs.