The "notify" (MCI_NOTIFY) flag directs the device to post an MM_MCINOTIFY message when the device completes an action. Your application must have a window procedure to process the MM_MCINOTIFY message for notification to have any effect. An MM_MCINOTIFY message indicates that the processing of a command has completed, but it does not indicate whether the command was completed successfully, failed, or was superseded or aborted.
The application specifies the handle to the destination window for the message when it issues a command. In the command-string interface, this handle is the last parameter of the mciSendString function. In the command-message interface, the handle is specified in the low-order word of the dwCallBack member of the structure sent with the command message. (Every structure associated with a command message contains this member.)