DITA coding practices for XML Schema rely on the XSD redefine facility in order to
implement specializations or constraints. However, limitations in the redefine facility
present problems for some DITA modules implemented in XML Schema.
An XSD structural module declares a top-level map or topic type. It is implemented
pair of XSD documents, one that defines groups used to integrate and override the
type and one
that defines the element types specific to the type.