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