7.2 - "abstract" and "concrete"

There is a slightly odd choice of terms in the first sentence of 7.2:

"Within XForms, XPath expressions reference abstract instance data (using the 
"path" portion of XPath), instead of a concrete XML document."

In what sense is the instance data (as defined in the XForms Glossary) 
"abstract"?

By "concrete" does the WD mean "serialised"?

Oh, in passing, what on earth is the "path portion" of XPath? That too seems 
a slightly odd choice of phrase. Why not just delete the parentheses and 
their content in the quote above?

Andrew Watt