Jon Udell talks about The Social Life of XML [Dec. 23, 2003]:

The really important thing, it seems to me, is the way the XML document can become a shared construct, a tangible thing that processes and people can pass around and interact with. On the one hand, an XML document is the payload of a SOAP message that gets routed around on the Web services network -- a payload that represents, for example, a purchase order. On the other hand, an XML document is the form that somebody uses to submit, or approve, or audit that purchase order. Now, all of a sudden, these two documents are not only made of the same XML stuff, they can literally be the same XML document.