This entry covers the ability to render only part of the scene. In the render tab, by checking the box labeled Border, this enables the ability to designate a rectangle within the camera view to restrict the rendered area (speeding up render times) if you are only working on a small part.
Once the box is checked, pressing [SHIFT+B] will enable a box select that will work in the camera view [NUM 0].
At this point, rendering [F12] will only render the area drawn in the red dotted box. If you check the Crop box, the black border representing the rest of the image won't be drawn.