The information items discussed so far (document, element, attribute, processing instruction, namespace declaration) all exist outside the realm of physical storage. In general, this is a good thing for most applications that consume XML. However, XML supports composing a single document information item from multiple source files or byte streams for modularity and flexibility. XML makes these underlying units of storage explicit via the notion of entities.
 Entities and DTDs are a holdover from the XML's roots in the documentation world. They are presented here largely for completeness. Their functionality is subsumed by XInclude, XBase, and XML Schemas, especially for software-based applications of XML.
Entities are units of ...