[Definition: A data object is an XML

　　document if it is well-formed,

　　as defined in this specification. In addition, the XML document is

　　valid if it meets certain further constraints.]

　　Each XML document has both a logical and a physical structure. Physically,

　　the document is composed of units called entities.

　　An entity may

　　refer to other entities to

　　cause their inclusion in the document. A document begins in a "root"

　　or document entity. Logically, the document

　　is composed of declarations, elements, comments, character references, and

　　processing instructions, all of which are indicated in the document by explicit

　　markup. The logical and physical structures nest properly, as described

　　in 4.3.2 Well-Formed Parsed Entities.