1. A method of using a second tag to extend a first tag in an XML document, the method including: receiving the XML document that includes the second tag defined in a second schema and a reference to the second schema; processing the second schema, which defines the second tag including a reference to the first tag defined in a first schema that is extended by the second tag; and an additional element from markup language that is not included in the first schema as part of the definition of the first tag and that extends the definition of the first tag to include the additional element; and processing the second tag as a polymorphic encoding of data in the XML document, such that the first and second schemas allow use of the first and second tags in the XML document; whereby applications and document types that use the first or second tag can evolve separately.
2. The method of claim 1 , further including parsing the XML document using a parser adapted to recognize the polymorphic encoding of the second schema and to parse the XML document and the second tag, even when business logic receiving the XML document is not adapted to process the additional element of the second tag.