Rendering to elements lets you separate various information in the rendering into individual image files. This can be useful when you work with some image-processing or special-effects software. You can later do compositing with the element renderings.
When you render one or more elements, a normal complete rendering is also generated. In fact, the element renderings are generated during the same rendering pass, so rendering elements costs little extra render time.
This is the Function Publishing Interface exposing the Render Element Manager:
The following is the list of the shipping Render Elements: