This macro is partially based on Steve Wright's Digital Compositing for Film and Video.
screenColor: You'll never find out what this is for...
limitingChannelMix: This slider is for setting the limiting color weights. For example: If the background is green, 0% limitingChannelMix means that green is limited by red, and 100% means green is limited by blue. 50% means, that it is limited by the average of red and blue, etc.
replaceImageMix: In most cases using the background image, or a replace image during spill suppression will help to get soft edges on the final composite. This parameter controls the mixing percent of the replacement image.
(The image will be mixed to the spill areas only. 100% admix won't mean a clean background, only when the original backing color was pure red, green or blue. Eventhough a non-clean mix can still help a lot for getting soft edges on the final comp.)
r/g/bShift: These controls are for increasing / decreasing any color component by shifting it's value in the spill areas. If you want the whole image to be affected, you can do the channel shift outside of this macro easily.