This part of ISO/IEC 19757 defines a language that is designed to extend the declarative functionality of an XMLDTD to include
— declaring one or more namespaces to which some or all of the element and attribute names in a DTD belong,
— declaring constraints on the content of elements with content model ANY to contain elements whose namesbelong to one or more specified namespaces,
— declaring datatypes for elements that contain data content only and for attribute values.
Two alternative syntax bindings for this language are defined. The first syntax binding uses XML processinginstructions and is designed to enable declarations in this language to be embedded within an XML DTD withoutinvalidating the DTD or altering its interpretation so far as legacy DTD parsers are concerned. This first syntax alsoprovides a means of associating a DTD with an external declarations subset containing declarations in either syntax.This syntax is defined in Clause 4 using the modified BNF syntax notation used in W3C XML.
The second syntax binding uses an XML document syntax and is defined in Clause 5. The syntax rules are definedby a schema that conforms to the RELAX NG Compact Syntax defined in ISO/IEC 19757-2. This syntax is designedto enable declarations in this language to be expressed almost entirely in XML (in this case one XML processinginstruction needs to be inserted in the DTD), to facilitate implementation using existing XML tools, either as anamespace-qualified fragment embedded within an XML instance or as a separate XML document.