An application must not share its registered classes with other applications. Some information in a window class, such as the address of the window procedure, is specific to a given application and cannot be used by other applications. However, applications can share an application global class. For more information, see Section 1.2.1.2, “Application Global Classes.”
Although an application must not share one of its registered classes with other applications, different instances of the same application can share a registered class. Once a window class has been registered by an application, it is available to all subsequent instances of that application. This means that new instances of an application do not need to, and should not, register window classes that have been registered by previous instances.