ScriptManagerProxy enables a content page to pass references to the ScriptManager placed on its ASP.NET master page. You can only have one script manager per page but you can have multiple proxies.
Used if you have the script manager in your master for instance which will go across all pages but you want a script manager reference in an underlying page or user control. In that instance you would add the proxy. If you added another script manager the page would error.
I hope it would make it clear :)
