XML Schema is a W3C initiative to provide a replacement for the existing document structure definition mechanism of XML, known as the Document Type Definition (or DTD), using XML markup itself instead of the idiosyncratic DTD notation. One of its principal official requirements was the provision of state-of-the-art mechanisms for definition structuring, composition and reuse, and these mechanisms in turn are intended to be modelled on, and a good impedance match with, existing structure definition systems from other branches of computer science, including programming languages and database systems.
As the above discussion of the Cambridge Communiqué suggested, the design of XML Schema makes explicit provision for annotation of the constituent parts, from which a schema is composed.