IBM® Integration Bus provides some additional facilities that are not specified in the XML Schema 1.0 specification.
A message is a global element that represents an entire message (rather than a structure within a message). Within an XML Schema file, a message is represented by a special global element that carries the extra information required by IBM Integration Bus.
For details about XML Schema, see XML Schema Part 0: Primer on the World Wide Web Consortium (W3C) website.