Specialization terminology

Certain terminology is used to discuss DITA specialization.

base type
An element or attribute type that is not a specialization. All base types are defined by the DITA specification.
extension element
Within a vocabulary module, an element type that can be extended, replaced, or constrained for use in a DITA document type.
The process by which a specialized element is transformed into a less-specialized ancestor element or a specialized attribute is transformed into a less-specialized ancestor attribute. The original specialization-hierarchy information can be preserved in the generalized instance; this allows the original specialized type to be recreated from the generalized instance.
(1) The act of defining new element or attribute types as a semantic refinement of existing element or attribute types
(2) An element or attribute type that is a specialization of a base type
(3) A process by which a generalized element is transformed into one of its more specialized element types or a generalized attribute is transformed into a more specialized attribute.
specialization hierarchy
The sequence of element or attribute types, from the most general to most specialized, from which a given element or attribute type is specialized. The specialization hierarchy for a DITA element is formally declared through its @class attribute.
structural type
A topic type or map type.