message listContainersRequest
parts 
parameters
element dir:listContainers
used by 
Operation listContainers in PortType DirectoryService
source  
<wsdl:message name="listContainersRequest">
  <wsdl:part name="parameters" element="dir:listContainers"/>
</wsdl:message>

WSDL documentation generated by XMLSpy WSDL Editor http://www.altova.com/xmlspy