The caller is a service notifying the user of an event. The function displays a message box on the current active desktop, even if there is no user logged on to the computer.

Terminal Services: If the calling thread has an impersonation token, the function directs the message box to the session specified in the impersonation token.

If this flag is set, the hWnd parameter must be NULL. This is so that the message box can appear on a desktop other than the desktop corresponding to the hWnd.

For information on security considerations in regard to using this flag, see Interactive Services. In particular, be aware that this flag can produce interactive content on a locked desktop and should therefore be used for only a very limited set of scenarios, such as resource exhaustion.