One advantage of an XML schema is its use as a formal specification language for the information model. For now, we'll continue to use the DTD format as our record definition of the model, and we'll rely on future tools to convert the DTDs to the forthcoming W3C XML Schema standard. We designed DITA in anticipation of the specialization principles behind schema languages, and we do plan to move to schema representation. A topic is a chunk of information organized around a single subject. Structurally, it is a title followed by text and images, optionally organized into sections.
Topics can be of many different types, the most common being concepts, tasks, and reference.
If we choose a smaller unit, the writer needs to check the unit in all its contexts to make sure that information flows correctly. A topic is large enough to be self contained from a writer's point of view but small enough to reuse effectively in whatever higher-level structure a particular delivery context requires. A topic can be augmented by a prolog that contains short descriptions and other optional metadata. Because each topic has a title and short description in addition to its full content, applications can provide progressive disclosure. For example, a user can hover over a link to see its short description and then decide whether to follow the link for the rest of the topic.