As described in the previous section an XML document is an entity that contains XML data of a particular type. The primary requirement is that a document have a root element, and XML defines some simple rules for data representation. To make sense of data represented in XML, it is necessary to know what the chosen element tags, etc. mean. This meaning is provided by a Document Type Definition (DTD) that defines what tags can be used and where they can be used. A unit of XML data that conforms to the rules defined in a DTD is an XML document