The Edge area includes the Use Edge Generator option, as well as fields to define the Edge Color, Edge Width, and Edge Generator Sensitivity.
Activate Use Edge Generator to make the Render window analyze color differences so that borders are added between shaded areas in your scene.
Use Edge Generator uses the color difference between two regions to determine if an edge is displayed. The Edge Generator Sensitivity value lets you adjust at which point the color difference creates an edge.
Use the Edge Generator Sensitivity field to determine the degree of color variance required for a border to be added between two shaded areas. A lower value in this field results in fewer edges being added to your scene.