In this post, Jeff Handley examines how attributes declared on your model are propagated to the client.
Source: Jeff Handley's Blog
Since the beginning of the RIA Services project, a primary tenet was to propagate validation rules defined on the server up to the client, integrating the validation directly into the Silverlight UI. This came to fruition with RIA Services v1.0 and you can in fact apply an attribute to your model on the server and automatically get instant client-side validation of data entry on even the simplest of Textbox controls. In fact, some validation rules are inferred from your data model directly. This is a feat that many other frameworks have attempted to accomplish, but I believe RIA Services has one of the most successful implementations of the concept.