3. A computer network system including at least one computer processor for processing electronic documents encoded in XML markup language, the computer network system comprising: a communications channel; at least one schema server in communication with the communications channel, wherein the schema server stores a first schema that defines a first tag which includes a plurality of elements from the XML markup language; and a second schema that defines a second tag; wherein the second schema includes: 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 wherein the second tag is polymorphically defined to be used to encode data within an XML document, such that the first and second schemas allow use of either the first or second tag in the XML document; whereby applications and document types that use the first or second tag can evolve separately.
4. The computer network system of claim 3 , further including an XML parser running on a computer, in communication with the communications channel, the parser being adapted to recognize the polymorphic encoding of the second schema and to parse the XML document and the second tag, even if business logic receiving the XML document is not adapted to process the additional element of the second tag.