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What does the Routing Parameter in API URL Query do ?
The Routing parameter was designed to allow backward compatibility and can be ignored.
eBay API WSDL file was switched from three-namespace (3 NS) to a single namespace (1 NS) configruation in e393. The Routing parameter was then introduced that allows third party application to tell Bay ApiServer to route the request to the single namespace code rather than the three-namespace code.
The Routing parameter is no longer required since all the eBay client applications have been migrated to the Unified Schema XML .
Documentation: Routing the Request