From W3C Wiki
Hybrid XHTML Documents: How Modularized XHTML Can Be Used to Create Compound Documents with Other XML-derived Markup Languages
Hybrid Document Types
Introduced in XHTML Modularization 1.1, a hybrid document type is a document type composed from a collection of XML document types or Modules. The primary purpose of the modularization framework described in XHTML Modularization 1.1 is to allow a document type author to combine elements from multiple abstract modules into a hybrid document type, develop documents against that hybrid document type, and to validate that document against the associated hybrid schema.
One of the most valuable benefits of using XML, is that XML reduces the barrier to entry for standardization of element sets that allow communities to exchange data in an interoperable format. However, the relatively static nature of HTML as the content language for the Web has meant that any one of these communities have previously held out little hope that their XML document types would be able to see widespread adoption as part of Web standards. The modularization framework allows for the dynamic incorporation of these diverse document types within the XHTML-family of document types, further reducing the barriers to the incorporation of these domain-specific vocabularies in XHTML documents.
Modularization & Validation
The use of well-formed, but not valid, documents is an important benefit of XML. In the process of developing a document type, however, the additional leverage provided by a validating parser for error checking is important. The same statement applies to XHTML document types with elements from multiple abstract modules.
An XHTML-family document is an instance of one particular document type as specified by the document's author, either in the document's prologue or through some other mechanism (for example, the schemaLocation attribute). Validating the document is the process of testing whether the document complies with the rules in the associated schema.
One document can consist of multiple document fragments. Validating only fragments of a document, where each fragment is of a different document type than the other fragments in the document, is beyond the scope of this framework - since it would require technology that is not yet defined.
However, the modularization framework allows multiple schema to be integrated and form a new document type (for example, SVG integrated into XHTML). The new schema can be used for normal XML 1.0 validation.