The MOM_POSITIONCB callback message notifies a client that a user-mode driver has encountered a MIDI event containing a MEVT_F_CALLBACK flag.
User-mode drivers do not send MOM_POSITIONCB callback messages.
Clients using the MIDI output stream functions send streams of data contained in a series of MIDIEVENT structures (see the Win32 SDK). Code in winmm.dll examines each MIDIEVENT structure and, if the MEVT_F_CALLBACK flag is set, sends a MOM_POSITIONCB message by calling DriverCallback, passing the specified parameters.
The winmm.dll code sends MOM_POSITIONCB messages only if the client has specified a notification target with the MODM_OPEN message.
Win32 SDK documentation states that clients receive a MM_MOM_POSITIONCB message if the notification target is a window handle. MOM_POSITIONCB and MM_MOM_POSITIONCB are equivalent.
For more information, see MIDI Output Streams, Notifying Clients from Audio Drivers and Transferring MIDI Output Data.