Section 9.4 says (trying to be helpful) "Also, the key and id functions can be used to find nodes within a temporary tree rooted at a document node, provided that at the time the function is called, the context item is a node within the temporary tree."

The "provided that" clause is obsolete and unnecessary since the 3rd argument of key() and the 2nd argument of id() were added to remove this restriction.

I would suggest simply deleting the clause starting "provided that...".