The Subsurface Scattering Render Element stores subsurface information from VRaySSS2 materials as an image. This render element can then be used to enhance or color-correct just the subsurface scattering component of the image during the compositing process.
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.