In addition to receiving messages, drivers must notify clients when certain events occur. MIDI output drivers notify clients by sending one of the following messages using the DriverCallback function.
| MOM_CLOSE | Notifies a client application when a device is closed. |
| MOM_DONE | Notifies a client application when the driver finishes with a system-exclusive data block and returns it to the client. |
| MOM_OPEN | Notifies a client application when a device is opened. |