A SOAP Fault Code that represents a SOAP element marked with the MustUnderstand attribute was not processed.
Not all SOAP elements require processing by the server. However, if a SOAP element is marked with the MustUnderstand attribute equal to 1 or true, processing is required. Failure to process the element generates a System.Web.Services.Protocols.SoapException with a SoapException.MustUnderstandFaultCode.