Specialization hierarchies are implemented as sets of module files that declare the markup and entities that are unique to each specialization. The modules must be integrated into a document type before they can be used.
The separation of markup into modules, as with the XHTML modularization initiative, (http://www.w3.org/TR/xhtml-modularization/ ), allows easy reuse of specific parts of the specialization hierarchy, as well as allowing easy extension of the hierarchy (since new modules can be added without affecting existing document types). This makes it easy to assemble design elements from different sources into a single integrated document type.
OASIS DITA Architectural Specification v1.0 -- 09 May 2005
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