The MOM_DONE callback message notifies a client that a user-mode driver has finished processing a MODM_LONGDATA message.
A user-mode MIDI output driver sends a MOM_DONE message to its client, by means of a callback, when the driver finishes processing a MODM_LONGDATA message. The driver sends the message to the client by calling DriverCallback, passing the specified parameters.
The driver sends the MOM_DONE message only if the client has previously specified a notification target with a MODM_OPEN message.
Win32 SDK documentation states that clients receive a MM_MOM_DONE message if the notification target is a window handle. MOM_DONE and MM_MOM_DONE are equivalent.
For more information, see Notifying Clients from Audio Drivers and Transferring MIDI Output Data.