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.