Because XML is widely used, DTDs and XML Schema are convenient ways to express standards. There are many such standards emerging in a variety of sectors. For example, the news and magazine publishing industries have developed NewsML [ITPC 2000] and PRISM (Publishing Requirements for Industry Standard Metadata) [PRISM 2001]. However, XML is merely convenient, it is not necessary. The DTDs and XML Schema say nothing about the semantics of the terms. Semantic agreements are implicit or informally captured in text documentation.