2.6.1 Recognized XML-document grammar mechanisms
The DITA standard recognizes three XML-document grammar mechanisms by which conforming DITA vocabulary modules and document types can be constructed: document type declarations (DTDs), XML Schema declarations (XSDs), and RELAX NG grammars.
This specification defines implementation requirements for all of these document grammar mechanisms. The OASIS DITA Technical Committee recognizes that other XML grammar languages might provide similar modularity and extensibility mechanisms. However, because the Technical Committee has not yet defined implementation requirements for those languages, their conformance cannot be determined.
Of these three document grammar mechanisms, RELAX NG grammars offer the easiest-to-use syntax and the most precise constraints. For this reason, the RELAX NG definitions of the standard DITA vocabularies are the normative versions. The DTD and XSD versions shipped by OASIS are generated from the RELAX NG version using open source tools.