How Windows Locates a Class

Windows maintains a list of structures for each of the three types of window classes. When an application calls the CreateWindow or CreateWindowEx function to create a window with a specified class, Windows uses the following procedure to locate the class:

Windows searches the list of application local classes for a class with the specified name. Note that several application instances can use the same name to register local classes. Windows uses instance handles to differentiate among local classes that have the same name.
If the name is not in the application local class list, Windows searches the list of application global classes.
If the name is not in the application global class list, Windows searches the list of system global classes.

All windows created by the application use this procedure, including windows created by Windows on the application's behalf, such as dialog boxes. It is possible to override system global classes without affecting other applications. That is, an application can register an application local class having the same name as a system global class. This replaces the system global class in the context of the application but does not prevent other applications from using the system global class.

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