When a ScriptManager control is already defined in a parent element (or master page or host page already contains a ScriptManager control) then used ScriptManagerProxy control to enables nested components and user control to add service and script. When you use the ScriptManagerProxy control, added to the script and service collections defined by the ScriptManager control.
If you do not used specific scripts and services on every page that includes a particular ScriptManager control. Add them to individual pages by using the ScriptManagerProxy control and remove those scripts and services from ScriptManager Control. The ScriptManagerProxy control requires specific settings in a web.config file in order to function correctly.