What this zone implies is that both calcium and alkalinity are elevated, and that by removing calcium carbonate from the water, either through biotic deposition into coral skeletons or coralline algae, or through abiotic precipitation, as on heaters, the levels of each will drop in an appropriate ratio.  More specifically, the tank parameters will move along a line parallel to the two lines bordering this zone, and directly into the red target zone (the blue arrow in the figure above).  If you are smack in the middle between these two lines, as in the figure, then you will continue to move in the middle of these two lines down into the target zone.