Hi Jzee I see no one answered on this. From what I've found out:
I think the Mode A & B affect the horizontal sync timing.
I found on Mode A, the image was centered on my display. On Mode B, the image was shifted to the left.
Its not the same for everyone and you can use the credits from a movie especially long ones where you will see the modes affect how the image is centrered. From some ppl a different mode causes a shift right instead of left etc, but essentially I believe is an aid to centre the picture on your display.
Hope this helps