1 - Spill Correction Opacity: This is the overall strength of the spill correction. This rarely needs adjust by very much. If the spill correction in the entire image needs adjusted then this is what you want to adjust. However, if the spill correction only needs adjusted for a certain color (yellow shirt for example) then the spill color range is what you will want to adjust.
2 - Spill Color Range: Important Concept to Understand - Spill color range is the range of color that are targeted for spill correction. This is different than the spill correction opacity. Spill color range sets the color thresholds where the spill correction starts to take effect.
For green screen, there are 2 adjustments, spill color range warms and spill color range cools. The spill color range warms affects how far the spill correction extends into the yellow spectrum. The spill color range cools affects how far the spill correction extends towards the yellow spectrum.
When green screen spill mixes with your image, it doesn't just turn areas green. It actually creates a gradient of colors in the foreground. For example, if a green screen with a hue of 120 mixes with hair that has a hue of 30, the spill creates a band of colors in the hair that range from 30-120. This entire band of colors needs shifted back to normal. This color range passes through the yellow spectrum and into the green spectrum. The spill color range is what defines where in this color range that the spill correction should start.
Watch the videos below which explains this in detail.