ORA-DM-ROOT-DOCUMENT

*Data Model, Section 4.2.1* says /"Document nodes and XPath 1.0 root 
nodes are essentially identical."/
If these terms are identical, why invent another term ?
If they are not identical, it would be good to have some indication of 
the differences here.
And if we must have another term, why call it a "document node" ? This 
will naturally be confused with the "document element", which in XPath 
1.0 is the child of the root node.