Gets the identifier of the CustomXmlPart which shall be used to evaluate the given XPath property. If specified, then the XPath expression specified in the XPath property shall only be evaluated against the custom XML part whose properties part has a matching identifier. If no custom XML part exists with a matching identifier, then the XML mapping shall not be connected. If value is null or empty, then the XPath expression shall be evaluated against each custom XML data part in turn until the given XPath expression is resolved to an XML node.