I had to ask the factory about this myself.

Guitar Mode is a feature designed specifically for Midi guitarists. They can play a chord and then press the sustain pedal which 'holds the chord' whilst the guitarists plays other stuff on the fret board, when he releases the sustain pedal he can then play another chord - its kind of like a MIDI hold/off function.

Mode 1 and Mode 2 are like a polarity switch for the sustain pedal which reverses the operation.

Hope that makes sense?