XMLPay From: http://www.verisign.com/rsc/wp/xml/xmlpay_spec/xmlpay_spec_wp.pdf Copyright VeriSign. A Document Type Definition (DTD) defines the structure of an XML document. With a DTD you can define the set and order of tags, as well as the attributes for each. A well-formed XML document is considered valid when it conforms to its corresponding DTD. The following is a Document Type Definition (DTD) representation of the XMLPay schema.