When you include a user control on your page, and specify that it should be cached (either by using the standard OutputCache directive in the .ascx file, or the PartialCachingAttribute in the the user control's code-behind file), an instance of the StaticPartialCachingControl class will be placed in the control hierarchy of the page as a parent to the cached user control.

public class StaticPartialCachingControl : BasePartialCachingControl {
// Public Constructors
   public StaticPartialCachingControl(string ctrlID, string guid, int duration, string varyByParams, 
        string varyByControls, string varyByCustom, BuildMethod buildMethod);
// Public Static Methods
   public static void BuildCachedControl(Control parent, string ctrlID, string guid, int duration, 
        string varyByParams, string varyByControls, string varyByCustom, BuildMethod buildMethod);
}

Hierarchy

System.Object Control(System.ComponentModel.IComponent, System.IDisposable, IParserAccessor, IDataBindingsAccessor) BasePartialCachingControl StaticPartialCachingControl