The Render Layer Mask creates a mask for each object in the scene (figure 1). This allows you to get the individual render pass of the desired object and control it as you like in a composition environment. This works like Cinema 4D's native Object Buffer.
Figure 1: The Render Layer Mask rollout in the Render Settings window.
Note: Render Layer Masks do not appear in the Live Viewer. They need to be rendered via the Picture Viewer.