This message is sent when a window being activated belongs to a different application than the currently active window. The message is sent to the application whose window will be activated and the application whose window will be deactivated.
| Parameter | Description |
| wParam | Specifies whether a window is being activated or deactivated. A nonzero value indicates that Windows will activate a window; zero indicates that Windows will deactivate a window. | |
| lParam | Contains the task handle of the application. If the wParam parameter is zero, the low-order word of the lParam parameter contains the task handle of the application that owns the window that is being deactivated. If wParam is nonzero, the low-order word of lParam contains the task handle of the application that owns the window that is being activated. The high-order word is not used. |