The VELO slider is a duplicate of the knob found in the Amp and Filter Envelope section of the Layer pages. This slider controls the effect of MIDI Velocity on the output of the Amp and Filter envelopes. Moving the slider to the right increases the dynamic sensitivity of the envelope and moving it to the left decreases it.
NOTE: Each of the VELO (Velocity) controls for the two AMP and two FILTER envelopes are independent. The VELO control will only affect the selected envelope.