Hi 
Folks,
 
[Thanks Len, you beat me to the mark.]
 
Peter, you make a good point, an XML document that is purely transient or 
purely persistent is likely the exception; the common cases are XML documents 
that are a mix of transience and persistence.
 
However, what I was trying to do was 
to explore the "space" of possibilities for XML usage.  To put it 
into semi-mathematical terms, I want to define the "axes/dimensions" of XML 
usage.
 
To summarize everyone's comments it appears that there are three 
"dimensions" to the usage of XML:
1. Persistent XML: the XML document is 
persistent.  Applications operate directly on the XML document.
2. Transient XML: upon arrival at its 
destination the data may be transformed into some other format (language 
objects, relational database, etc) that applications work with.
3. Application XML: the XML document 
is the application. 
Question:
 
Does the usage (role) of an XML document influence its design?  
 
For example, are transient XML documents typically flat, whereas persistent 
XML documents typically nested?
 
Peter, I am still struggling how to put into the above "space" 
your ideas on XML-and-UI.  Your assertion is that the usage of XML is not a 
3-dimensional space, but a 4-dimensional space?  Can you characterize the 
fourth dimension?
 
/Roger