xmlDoc

The 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.

xmlDoc's can be saved to a file and converted to strings with the xmlTreeSaveDoc() (here) and xmlTreePrintDoc() (here) functions, respectively.

Also see the xmlDoc vs. Root Element section (here) for details on the differences between xmlDoc's and the root element of a document.