Specialization allows you to define new kinds of information (new structural types or new domains of information), while reusing as much of existing design and code as possible, and minimizing or eliminating the costs of interchange, migration, and maintenance.
Specialization is used when new structural types or new domains are needed. DITA specialization can be used when you want to make changes to your design for the sake of increased consistency or descriptiveness or have extremely specific needs for output that cannot be addressed using the current data model. Specialization is not recommended for simply creating different output types as DITA documents may be transformed to different outputs without resorting to specialization (see Customization).
There are two kinds of specialization hierarchy: one for structural types (with topic or map at the root) and one for domains (with elements in topic or map at their root). Structural types define topic or map structures, such as concept or task or reference, which often apply across subject areas (for example, a user interface task and a programming task may both consist of a series of steps). Domains define markup for a particular information domain or subject area, such as programming, or hardware. Each of them represent an “is a” hierarchy, in object-oriented terms, with each structural type or domain being a subclass of its parent. For example, a specialization of task is still a task; and a specialization of the user interface domain is still part of the user interface domain.
Use specialization when you are dealing with new semantics (new, meaningful categories of information, either in the form of new structural types or new domains). The new semantics can be encoded as part of a specialization hierarchy, that allows them to be transformed back to more general equivalents, and also ensures that the specialized content can be processed by existing transforms.
OASIS DITA Architectural Specification v1.0 -- 09 May 2005
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