The prolog is considered to be optional in an XML document. If it is included in the document, it must be placed before the root element. A prolog has one or both of two items: the XML declaration and the DTD. If both are used, then the XML declaration should be placed before the DTD declaration.
The XML declaration is where the version of XML being used is defined. The current version is 1.0 and this can be verified at the w3.org site. The declaration is not necessary, but it is a good idea to include it. And example of an XML declaration is the following:
Example: <?xml version=”1.0″?>