The !ELEMENT Statement
Every element used in your XML documents has to be declared by using the <!ELEMENT> tag in the DTD. The format for declaring an element in a DTD is shown here:
<!ELEMENT ElementName Rule> |
The Rule component defines the rule for the content contained in the element. These rules define the logical structure of the XML document and can be used to check the document's validity. The rule can consist of a generic declaration and one or more elements, either grouped or unordered.