The MIM_LONGDATA callback message notifies a client that a user-mode driver has received a MIDI system-exclusive (long) message.
A user-mode MIDI input driver sends a MIM_LONGDATA message to its client, by means of a callback, when the driver has received a MIDI system-exclusive (long) message. The driver sends the message to the client by calling DriverCallback, passing the specified parameters.
The driver sends the MIM_LONGDATA message only if the client has previously specified a notification target with a MIDM_OPEN message.
If the driver detects that the message is invalid, it should send MIM_LONGERROR instead of MIM_LONGDATA.
Win32 SDK documentation states that clients receive a MM_MIM_LONGDATA message if the notification target is a window handle. MIM_LONGDATA and MM_MIM_LONGDATA are equivalent.
For additional information, see Notifying Clients from Audio Drivers and Transferring MIDI Input Data.