Some interesting points here.

> An XML schema is a physical data model.  It is physical 
> because it includes significant amounts of information 
> specific to a particular physical representation of the data, 
> e.g. you have to worry about elements versus attributes in an 
> XML schema.

Well, I think the original ANSI-SPARC terminology was different. The logical
level was what applications dealt with, the physical level was the way it
was encoded on disk, and the conceptual model was the way people thought
about it.

> However, you 
> wouldn't normally expect to factor out "Person" as the common 
> superclass; that's the kind of technical construct that is 
> appropriate for a logical model, but often inappropriate for 
> a conceptual model.

I think it's very common, and very beneficial, to find abstractions like
this being created in the business domain rather than the technical domain.
For example, legislation will talk about "vehicles" rather than cars,
bicycles, and lorries; HR people will have a term that covers employees and
full-time contractors; and finance people will lump together cash and
buildings as "assets". It's true that the technical design might identify
further abstractions (for example treating phone numbers, email addresses,
and postal addresses as examples of something more general) - but these are
far less valuable than those that originate in the business, because you
can't build common business processes around them.

Michael Kay
http://www.saxonica.com/