The MIM_OPEN callback message notifies a client that a user-mode driver has finished processing a MIDM_OPEN message.
A user-mode MIDI input driver sends a MIM_OPEN message to its client, by means of a callback, when the driver finishes processing a MIDM_OPEN message. The driver sends the message to the client by calling DriverCallback, passing the specified parameters.
The driver sends the MIM_OPEN message only if the client has specified a notification target with the MIDM_OPEN message.
Win32 SDK documentation states that clients receive an MM_MIM_OPEN message if the notification target is a window handle. MIM_OPEN and MM_MIM_OPEN are equivalent.
For additional information, see Notifying Clients from Audio Drivers and Transferring MIDI Input Data.