The Service segment of the WSDL specifies the web service used, which, in this case, is Product Advertising API, as shown in the following WSDL snippet:

<service name="AWSECommerceService">
  <port name="AWSECommerceServicePort" 
    binding="tns:AWSECommerceServiceBinding">
    <soap:address location= "http://soap.amazon.com/onca/soap?Service= AWSECommerceService" /> 
  </port>
</service>

This information changes very rarely and so you need not pay much attention to it.

Every Product Advertising API request includes this service declaration, as shown in the following example.

http://ecs.amazonaws.comonca/xml?
Service=AWSECommerceService&
Operation=ItemSearch&
SearchIndex=Books&
Keywords=Saving%20Miss%20Oliver's