Although you could create a new elementequivalent for any tag in a general DTD, this work is useless to you as an author unless the content models that would include the tag are also specialized.
DITA instead leans toward a different SGML reuse technique and provides each element with a conref attribute that can point to any other equivalentelement in the same or any other topic.
The W3C XML Schema allows for a class of element types that generally appear in the same locations to be treated as a group of equivalent elements in type definitions.