Artists have for thousands of years been separating the lightness values of their paintings into three bands: shadows, midtones and highlights. The ColorCorrect node is a collection of operations that not only can effect the entire image but can address separately these three ranges (below, with the master parameters as well as the three tone ranges).
The manner in which these values are separated can be set by moving the curves around in the Ranges tab (below).
Click the test parameter box to get visual feedback on these ranges, which will be presented as white, grey and black with the cusp (soft in-between) regions as green (below).
Tip: this node should not be used without checking the ranges first.
Tip: when any adjustment to the default settings are made be sure to keen an eye of the cusp values. If the three tone regions meet too sharply and are subject to extreme adjustment, then unpleasant artifacting can result.