Configuration, specialization, and constraints

In this section:

1. Overview of DITA extension facilities
   DITA provides three extension facilities: configuration of the vocabulary modules used by DITA document types, constraint of base content models and attribute lists, and creation of new element and attribute types (specialization).
2. Configuration (Document type shells)
   A given DITA map or topic document is governed by a DITA document type that defines the set of structural modules (topic or map types), domain modules, and constraints modules that the map or topic can use.
3. Specialization
   The specialization feature of DITA allows for the creation of new element types and attributes that are explicitly and formally derived from existing types. The resulting specialization allows for the blind interchange of all conforming DITA content and a minimum level of common processing for all DITA content. It also allows specialization-aware processors to add specialization-specific processing to existing base processing.
4. Constraints
   Constraint modules define additional constraints for corresponding vocabulary modules in order to restrict content models or attribute lists for specific element types, remove extension elements from an integrated domain module, or replace base element types with domain-provided extension element types. Constraint modules do not and cannot change element semantics, only the details of how element types can be used in the context of a specific concrete document type. Because constraints can make optional elements required, documents that use the same vocabulary modules may still have incompatible constraints. Thus the use of constraints can affect the ability for content from one topic or map to be used directly in another topic or map.