ScriptControl inherits from the WebControl class and implements the IScriptControl interface. The WebControl class is a base class for ASP.NET Web server controls. The ScriptControl is an abstract class, which you cannot instantiate directly. Instead, you derive a class based on this abstract class.
The ScriptControl base class tests the page for a ScriptManager control during the PreRender stage of the derived control. The ScriptControl base class also makes sure that the derived control calls methods of the ScriptManager control to register script during the Render event. This includes registering ScriptDescriptor objects for the ScriptControl when the Render method is called. The Render method makes sure that ScriptDescriptor objects are not rendered unless the ScriptControl itself is rendered. This enables a ScriptControl to work inside a closed WebPart control.
You can add as many descriptors and types as necessary to any class that you derive from ScriptControl.