Mamiya C220 Cds Prism Dial Question

I have cds metered prism for my C220 camera. On the settings dial there are three scales. The one in the center is for the ASA/DIN setting, the second runs across the top is the exposure setting. The third scale on along the bottom has markings for 6.3 to 2.8 and is detented with several stop points. What is this third dial for?

Read here: http://dotinthelandscape.org/mfaq/mfaq/mfaq06/#64-finders
section 6.4.5 and 6.4.7.

You have to set on the meter the maximum aperture of the lens, because it is metering through the viewing lens, and there is no linkage for the lens to tell the meter what aperture of lens it is looking through.

Makes sense... Thanks!!!