xmlDoc is a special case of
xmlNode. Each XML
document has a single
xmlDoc that is that the root of the
document. It can be used in many of the same functions as
xmlNode's (such as
xmlTreeGetChildren()), but also contains
document-wide information, such as character encoding.
Also see the
xmlDoc vs. Root Element section
(here) for details on the differences between
xmlDoc's and the root element of a document.