A Web page can contain only one ScriptManager control, either directly on the page or indirectly inside a nested or parent component. The ScriptManagerProxy control lets you add scripts and services to content pages and to user controls if the master page or host page already contains a ScriptManager control.
When you use the ScriptManagerProxy control, you can add to the script and service collections defined by the ScriptManager control. If you do not want to include specific scripts and services on every page that includes a particular ScriptManager control, you can remove them from the ScriptManager control. You can then add them to individual pages by using the ScriptManagerProxy control instead.
ScriptManagerProxy control enables a user to add scripts and services that are specific to nested components. If a page already contains the ScriptManager control. Only one instance of the ScriptManager control can be added to a page. The page can include the control directly or indirectly inside a nested component such as a user control or nested master page. The ScriptManagerProxy control is used when the ScriptManager control is already in the page and a nested or parent component requires additional features of the ScriptManager control.
<asp:ScriptManagerProxy ID="ScriptManagerProxy1" runat="server"></asp:ScriptManagerProxy>
Main Property of ScriptManagerProxy Control is AuthenticationService, ProfileService, Scripts, and Services etc