When an application creates a window with a specified class, Windows uses the following procedure to find the class:
1.Windows searches for a local class of the specified name.
2.If Windows does not find a local class with the name, it searches the application global class list.
3.If Windows does not find the name in the application global class list, it searches the system global class list.
This procedure is used for all windows created by the application, including windows created by Windows on the application's behalf, such as dialog boxes. It is possible, then, to override system global classes without affecting other applications.