The Matte Curves node displays the same menu that is displayed when you select the Result node—the Matte Curves menu. Use this menu to adjust the luminance of the front and back mattes:
You can add a Matte Curves node to any part of the pipeline except to the CBlend and MBlend pipes. This is because each layer of the blend nodes has its own Matte Curves menu built into it. To learn about adjusting the front and back mattes of blend node layers, see Blending the Front and Back Images of a Layer.
See Result Node for instructions on using the Matte Curves menu.