Each element declared in the DITA architecture has a class attribute. This attribute provides a mapping between the element's current name and its more general equivalents. The more specialized the element type, the longer its class attribute value.
<!ATTLIST step class CDATA "- topic/li task/step ">
This tells us that the step element is equivalent to the li element in a generic topic. It also tells us that step is equivalent to a step in a task topic, which we already knew, but it's worth noting this in the attribute because it enables round-trip migration between upper level and lower level types without loss of information. For example, if a user runs a "generalize" transform that maps all elements to their first class value, but preserves their content and attribute values, then the user can follow it up with a "specialize" transform that maps all elements to their last class value (preserving content and attribute values), and provide a full round trip for all content between the two document types, using nothing but two generic transforms and the information in the class attribute.
The class attribute tells a processor what general classes of elements the current element belongs to. It's something like an architectural forms attribute, except that it contains multiple mappings in a single attribute, instead of one mapping per attribute. Also, DITA scopes values by module type (for example topic type, domain type, or map type) instead of document type, which lets us combine multiple topic types in a single document without complicating transform logic.
OASIS DITA Architectural Specification v1.0 -- 09 May 2005
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