This short clip from Ableton's Learn Live series covers MIDI mode in Emit, the granular device from the Inspired by Nature Pack. When MIDI mode is active, particles are generated by played notes rather than triggered automatically, giving you rhythmic and melodic control over grain playback.
By default, the pitch of each MIDI note maps directly to the pitch of the particles it creates. That mapping can be disabled so only the rhythmic pattern of the notes affects playback, leaving pitch behavior to Emit's own settings.