Data Model - 4. Nodes

The final part of Chapter 4, Nodes, reads as follows:
"Every node belongs to exactly one tree, and every tree has exactly one root 
node. A tree whose root node is a document node is referred to as a document. 
A tree whose root node is some other kind of node is referred to as a 
fragment.".

Since XPath/XQuery operate on the infoset and the infoset is only defined for 
the scenario where there is a document node since according to the Infoset 
Rec it applies only to well-formed XML documents, I don't see where these 
sentences are leading.

Andrew Watt