The website explains it. Here it is from link I gave you: "The so called "Bolt-area" indicates a accumulation of good room ratios. If the red cross lies within this area you have a good chance of more evenly distributed modes."
Put in different dimensions into the calculator and you will see the X move into or out of the Bolt area. Good luck!
I did not see that anywhere I looked, and with the axes being unlabelled, so it to seems like a rhorshack drawing.
I want to understand it though.