Client-side rendering allows a report to be fully rendered, exported and printed on the client. In Silverlight based scenarios, this reduces the server load and offers the possibility of creating reports when the data is located in the client (e.g. RIA Services, OData, or any plain REST service).
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Fully render reports in the client and reduce server load.  

The Windows Communication Foundation (WCF) service processes the reports on the server, sending only the final result to the client.
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Report Service