VBA Code

Dim objDOM As DOMDocument40 
Dim sXML As String 

sXML = Application.Selection.Range.WordOpenXML 
objDOM.LoadXML(sXML) 
objDOM.Save("C:\temp\filename") 

DocumentFormat.OpenXML ? 

Open XML SDK 2.0 productivity Tool for Microsoft Office

Office.CustomXMLPart
Office.CustomXMLNodes
Office.CustomXMLNode

Office.msoCustomXMLNodeType