1. A computer network system for processing electronic documents encoded in XML markup language, the computer network system comprising: a communications channel; a first server, the first server being in communication with the communications channel, wherein the first server stores a first schema that defines a first tag which includes a plurality of elements from the XML markup language; a second server, the second server being in communication with the communications channel, wherein the second server stores a second schema that defines a second tag including a reference to the first tag defined in the first schema, wherein the reference identifies the first tag as being extended; and an additional element from the XML markup language that is not included in the first schema as part of the definition of the first tag; wherein the second tag is polymorphically defined to be used to encode data within an electronic document, such that the first and second schemas respectively allow use of the first or second tag in the electronic document; whereby applications and document types that use the first or second tag can evolve separately.