[Paraview] how does ScalarOpacityUnitDistance affect volume rendering?

[Jeff Mauldin]

I am working with volume rendering on a particular data set.  I am doing 
projected tetra volume mapping.  When I bring up the Color Scale Editor 
dialog (which I have used extensively), there is an entry called "Scale" 
that I haven't been able to find documentation to.  In playing with the 
python tracing, I see that it alters a value called 
ScalarOpacityUnitDistance.  The effect appears to be that setting this 
parameter to zero causes the rendering to be completely invisible (alpha 
zero everywhere, presumably) and setting it to 1 causes the rendering to 
be completely opaque.  I'm guessing it's some kind of multiplier to the 
opacity value in color transfer function, and possibly some kind of 
multiplier applying to how much each volume ray segment contributes to 
the overall opacity of a pixel.  What exactly is this scale value doing 
(so I can understand what I'm looking at)?