Creates faces (to facet) to fill a space surrounded by selected vertices usng either the Actions > Fill Outline or Actions > Fill to Cursor commands.
Auto Filling is by far the easiest way to facet an object, but it is not always appropriate. To use it, just draw an outline curve with the Pen Tool, remembering to close it, (the Pen Tool will close it if you click exactly on the first vertex. Then, with all the vertices of the curve selected, center the 3D Cursor and choose Fill to Cursor from the Actions menu. The curve should now be "solid". You can check this by making a enabling OpenGL shading in the Camera View. The OpenGL Shaded Camera View will only display filled outlines with faces, not unfilled outlines.
Auto Filling, although easy to activate, has some limitations. The main limitation is that only convex shapes can be faceted in one operation. It is possible to facet a concave outline with Auto Filling, but be very careful how the 3D Cursor is placed.