Service client API has objects for request and response data. Contrasting old way of retrieving things using the AWS SDK with the new way, it looks like they have switched from SOAP API to a RESTful API. The old way requires you to create Request and Response objects every time you want to do anything with the AWS API. Looking into the implementation, this looks a lot like SOAP.
The new way is a lot more neater. A request can be built using builders that work on a conceptual model of the request. Response is also a lot more conceptual and easy to read.