You use this method to register a startup script block that is included every time that an asynchronous postback occurs. To register a script block for a control that is inside an UpdatePanel control so that the script block is registered only when the UpdatePanel control is updated, use the RegisterStartupScript(Control, Type, String, String, Boolean) overload of this method.
If you want to register a startup script that does not pertain to partial-page updates, and if you want to register the script only one time during initial page rendering, use the RegisterStartupScript method of the ClientScriptManager class. You can get a reference to the ClientScriptManager object from the ClientScript property of the page.