Defining the structure of an XML document must always involve the declaration of elements because all XML documents contain at least one element: the document element.
An element declaration takes this general form:
<!ELEMENT elementTypeName contentModel >
The allowed content models are listed here:
Text only— Indicated by (#PCDATA). No element content is allowed.
Empty element— Indicated by EMPTY. Not even whitespace is allowed as content.
Any content— Indicated by ANY. Any well-formed content is allowed.
Mixed content— Indicated by MIXED. This allows text content to be mixed with element content declared in the element declaration.
Child elements— Indicated by one or more element names contained in parentheses, ...