Page "Document Type Definition" Paragraph 34

Some applications ( but not XML or SGML parsers themselves ) also allow referencing notations indirectly by naming them in the < tt >" URN: name "</ tt > value of a standard CDATA attribute, everywhere a URI can be specified.

However this behaviour is application-specific, and requires that the application maintains a catalog of known URNs to resolve them into the notations that have been parsed in a standard SGML or XML parser.

This use allows notations to be defined only in a DTD stored as an external entity and referenced only as the external subset of documents, and allows these documents to remain compatible with validating XML or SGML parsers that have no direct support for notations.