Example Service Message
        All service messages extend the ServiceMessage complexType (defined in the Service.xsd schema).
The Agent Action messages then include Agent Header information (MessageHeader) followed
            by an Agent Action, in this example ShipmentAction. All valid Shipment Actions are
            defined in the ShipmentAction.xsd schema. The following example shows the Set Indicator
                Action:
The SetIndicator element is allowed to be substituted for the ShipmentAction element
            because it is defined as an XSD substitutionGroup and extends the
            same AgentActionType complexType.