Architectural specification : Architectural Specification: Base : Configuration, specialization, and constraints : Overview of DITA extension facilities : Recognized XML document constraint mechanisms
   
Recognized XML document constraint mechanisms
The DITA standard currently recognizes two XML document grammar mechanisms by which conforming DITA vocabulary modules and document types may be constructed: document type declarations (DTDs) and XML Schema declarations (XSDs).
This specification defines implementation requirements for both of these document constraint mechanisms. The OASIS DITA Technical Committee recognizes that other XML grammar languages might provide similar modularity and extensibility mechanisms. However, the Technical Committee has not yet defined implementation requirements for those languages so their conformance cannot be determined.