When you open Compositor View with the (RE) option, the schematic contains, in addition to the standard Output Bitmap nodes, an Output Bitmap node for each render element active within a state.
As well, the Compositor Output node contains an extra layer for each render element node. Each render element node's name has the type inserted. For example, if a state's name is Trees, and you've enabled a Lighting render element in that state, the main rendered bitmap node for the state will be named Trees Output and the render element bitmap node will be named Trees Lighting Output.
The render element nodes don't receive special treatment; they function basically the same as regular Bitmap Output nodes. It's up to you to determine how to use them in your compositor application.