Equivalence of document-type shells

Two distinct DITA document types that are taken from different tools or environments might be functionally equivalent.

A DITA document type is defined by the following:

  • The set of modules that are declared in the @domains attribute on the root element of the document
  • The values of the @class attributes of all the elements in the document
  • Rules for topic nesting

Two document-type shells define the same DITA document type if they integrate identical vocabulary modules, constraint modules, and rules for topic nesting. For example, a document type shell that is an unmodified copy of the OASIS-provided document-type shell for topic defines the same DITA document type as the original document-type shell. However, the new document-type shell has the following differences:

  • It is a distinct file that is stored in a different location.
  • It has a distinct system identifier.
  • If it has a public identifier, the public identifier is unique.


The public or system identifier that is associated with a given document-type shell is not, by itself, necessarily distinguishing. This is because two different people or groups might use the same modules and constraints to assemble equivalent document type shells, while giving them different names or public identifiers.

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