Applications must not share registered classes with other applications. Some information in a window class, such as the address of the window function, is specific to a given application and cannot be used by other applications. However, applications can share an application global class. See “Application Global Classes,” in Section 1.2.1 for more information.
Although applications must not share registered classes, 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.