For one thing it's actually suppress_notify and not supress_notify. At one point it was supress_notify but Mark corrected the typo.

notify in the output section tells the player to send out information on the serial port about what it's playing and what point it is at in the song.
suppress_notify in the hijack section however tells Hijack to silently absorb those messages and not send them out on the serial port. The messages are still parsed by Hijack however.