Apple Color 1.0 - Manual (Page 185)

Chapter 9 Primary In 185
 If Limit Shadow Adjustments is turned off: Color adjustments made using the shadows
control are performed as a simple add operation (the color that’s selected in the
Shadow color control is simply added to that of every pixel in the image). The
resulting correction affects the entire image (and can be seen clearly within the
gradient at the bottom of the image), producing an effect similar to a tint.
 If Limit Shadow Adjustments is turned on: A linear falloff is applied to color
adjustments made with the Shadow control such that black receives 100 percent of
the adjustment and white receives 0 percent of the adjustment. This is the method
to use if you want to be able to selectively correct shadows while leaving highlights
untouched.