Document Type Definition. The DTD is an optional part of the prolog of an XML document that defines the rules of the document. It sets constraints for an XML document by specifying which elements are present in the document and the relationships between elements, e.g., which tags can contain other tags, the number and sequence of the tags, and attributes of the tags. The DTD helps to validate the data when the receiving application does not have a built-in description of the incoming data.