The base DITA element and attribute types may be extended through the creation of new vocabulary modules that define specializations of more-general types.
2.1.4.3.8.1 DTD syntax specialization module coding requirements
To be extensible and backward compatible, DITA requires that a DTD implementation of structural and domain specialization modules conform to well-defined implementation (coding) requirements.
2.1.4.3.8.2 XSD schema specialization module coding requirements
To be extensible and backward compatible, DITA requires that an XSD implementation of structural and domain specialization modules conform to well-defined implementation (coding) requirements.