For what is "parameterOrder" in a WSDL?

Assume the following part of a WSDL definition:

  <message name="Ws_myRequest">
    <part element="tns:myRequest" name="myRequest"/>
  </message>
  <portType name="Ws">
    <operation name="myWs" parameterOrder="myRequest">
      <input message="tns:Ws_myRequest"/>
      <output message="tns:Ws_myResponse"/>
    </operation>
  </portType>


For what stands the parameter "parameterOrder" ?
as the message "Ws_myRequest" shows there is only 1 parameter.
So can I omit this "parameterOrder"?

What if I would have two elements like in:

  <message name="Ws_myRequest">
    <part element="tns:myRequest1" name="myRequest3"/>
    <part element="tns:myRequest2" name="myRequest4"/>
  </message>

How could the "parameterOrder"  look like in this case?

Ben