Section 508 Reference Guide - Description of Figure 1
Figure 1 illustrates two views from above. In both views, the vertical plane is centered on the control area. ("CL" represents the centerline of the control area.) In the first view, the vertical plane is set back from the control area by a protrusion on the device. In the second view, there are no protrusions on the device and the vertical plane is right up against the control area.
[figure and description adapted from the Access Board]