A document type shell is an XML grammar file that specifies the elements and attributes that are allowed in a DITA document. The document type shell integrates structural modules, domain modules, and constraint modules. In addition, a document type shell specifies whether and how topics can nest.
A DITA document must either have an associated document-type definition or all required attributes must be made explicit in the document instances. Most DITA documents have an associated document-type shell. DITA documents that reference a document-type shell can be validated using standard XML processors. Such validation enables processors to read the XML grammar files and determine default values for the domains and class attributes.
The following figure illustrates the relationship between a DTD-based DITA document, its document-type shell, and the various vocabulary modules that it uses. A similar structure applies to DITA documents that use other XML grammars.
The DITA specification contains a starter set of document-type shells. These document type shells are commented and can be used as templates for creating custom document-type shells. While the OASIS-provided document-type shells can be used without any modification, creating custom document-type shells is a best practice. If the document-type shells need to be modified in the future, for example, to include a specialization or integrate a constraint, the existing DITA documents will not need to be modified to reference a new document-type shell.