This determines how much the foot controller will affect the overall volume
of the sound. This is like the other amplitude-modifying parameters in that
in works in the opposite manner than you would expect. My guess is that the
TX81Z has a certain maximum amplitude that it can output, so this setting
decreases the volume of the pedal's zero point. In other words, the higher
the setting is, the lower the volume is when the foot controller is at zero,
which effectively gives you more control of the volume.