The Matte Shadow Render Element shows areas where light doesn't reach. Compare with the Raw Shadow Render Element, which gives areas of cast shadows. While technically not part of the Beauty render elements, the Matte Shadow Render Element can be used to lighten cast shadows by adding it to the Raw Lighting Render Element. It can also be used to assist in adding 3D elements to a background image or footage in the composite.
To use render elements, select from the Available render elements column and click the Add button to add the passes you wish to generate at render time. When you render the scene, the added render elements listed in the column on the right, will also render, which can be viewed from the VFB window's channel drop-down menu.
For more information on the parameters on this dialog, see the Render Elements tab page.
