What is the difference between root node, root element and document element in XML?

Question

I am reading W3C XML Path Language (XPath) Version 1.0 Section 5.1 Root Node,

The root node is the root of the tree. A root node does not occur except

Answer 1

In your XML:

• a is the document element, sometimes called the root element, which is selected via /a.
• The parent of a is the document's root node, which is selected via /.