1. Press the Arpeggiator button, then press the >> button 9 times to get to the Vel Mode parameter. Change the value to Ctrl 117.

2. Hold the Controllers button and move Slider C. Press 117, then Enter to assign Arp Vel for the slider. (You might want to check the Scale, Offset, and Curve parameters for Slider to make sure they are at their defaults of 100%, 0, and Linear).

3. Now hold some keys and move the slider up and down. The arpeggiated notes increase and decrease in volume. The slider controls the velocities used for the notes created by the arpeggiator. You could use this effect to fade arpeggiated notes in with an non-arpeggiated layer.

You could create a similar end result by assigning the slider to MIDI volume for each zone that uses the arpeggiator. However, there is a difference. If a single program switched between different samples based on the velocity of the note, then using the slider to control velocity instead of volume would let you switch between those different samples in the arpeggio.