A recent discussion on xml-dev list motivated me to know more about NVDL.
NVDL stands for, Namespace-based Validation Dispatching Language.
NVDL is Part 4 of ISO/IEC 19757 DSDL (Document Schema Definition Languages).
If an XML document is composed of sections belonging to multiple namespaces, then each section (differentiated by the namespace) can be dispatched for validation to a different schema processor (for e.g., RELAX NG, W3C XML Schema, DTD, Schematron etc.).
This is an innovative new idea to allow validation of different parts of XML document with different schema languages.
This is what Roger Costello wrote about NVDL on xml-dev list:
Here are the evolutionary changes I envision NVDL bringing about in the marketplace:
1. Opens the marketplace to utilizing a variety of schema languages.
Previously, you and all your trading partners were locked into using one schema language (typically W3C XML Schema) if you wanted interoperability. With NVDL that limitation is lifted and you can achieve interoperability while using a variety of schema languages.
2. Promotes using the right schema language for the right job.
XML Schema and Relax NG are two schema languages for expressing grammar-based rules. They are both standards, the former a W3C standard, the later an ISO standard. Although their capabilities are largely overlapping, there are important differences. "Use the right tool for the right job" is an adage that applies to choosing a schema language. Knowing the differences in capabilities is important to making a good decision in choosing a schema language.
3. Encourages the creation of small, simple, independent schemas, written in any schema language.
4. Moves the application developer's focus from:
"using a schema"
"using XML vocabularies"