This operator is implemented in VEX, which supports parameter overloading. If the first input has a plane which matches an operation parameter’s channel name, the input plane will be used as the parameter’s value, effectively overriding it. The overloaded parameter is then evaluated from the plane on a per-pixel basis.

eg. If the input COP has the following planes:

C{r,g,b}
A
fogdens

and it is fed into a VEX Fog COP, the fog density will be determined at each pixel by the fogdens plane, since the Fog Density parameter’s channel name matches the 'fogdens' channel name.