The structure of an XML document is defined by a DTD
(Document Type Definition), an XML specification based
on suitable meta-tags; a document that respects a specific
DTD is said to be valid. Consequently, a DTD induces a
class of documents, i.e. the class of all valid documents
with regard to the specific DTD.
Distributed applications share the same DTD schema.
DeMers, M.N. (2002) Fundamentals of GIS , 2nd edn, John Wiley & Sons.