The ScriptManagerProxy control enables nested components such as content pages and user controls to add script and service references to pages when a ScriptManager control is already defined in a parent element.
A Web page can contain only one ScriptManager control, either directly on the page itself or indirectly inside a nested or parent component. The ScriptManagerProxy control enables you to add scripts and services to content pages and to user controls where 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, remove them from the ScriptManager control and add them to individual pages by using the ScriptManagerProxy control instead.
The ScriptManagerProxy control requires specific settings in a web.config file in order to function correctly. If you try to work with this control, and your website doesn't contain the required web.config file, errors appear in the Design view of the page where the control would have appeared. In Design view, if you click the control that is in that state, Microsoft Expression Web will give you the option to create a new web.config file or update your existing web.config file.
For more information about the ScriptManagerProxy control, see ScriptManagerProxy Class in the MSDN Library.