wsdl namespaces in Axis

[MKW <m....@ru.ac.za>]

Please can someone explain the double namespace that appears in the 
following Axis-generated wsdl fragment:

wsdl:portType name="MyPort">
       <wsdl:operation name="tns.storeEntryInDB">
          <wsdl:input message="impl:tns.storeEntryInDBRequest" 
name="tns.storeEntryInDBRequest"/>
          <wsdl:output message="impl:tns.storeEntryInDBResponse" 
name="tns.storeEntryInDBResponse"/>
       </wsdl:operation>
       <wsdl:operation name="tns.returnEntry">
          <wsdl:input message="impl:tns.returnEntryRequest" 
name="tns.returnEntryRequest"/>
          <wsdl:output message="impl:tns.returnEntryResponse" 
name="tns.returnEntryResponse"/>
       </wsdl:operation>

Why do the inputs and outputs have "impl:tns." as a prefix?  I've looked 
for this construct on namespace FAQs and can't find it.  It looks like a 
mix of a namespace and a package???  The target namespace and the "impl" 
or implementation namespaces actually refer to the same place.

Thanks.