Runout vs. Flatness and Dial test indicators

There is a section in the 7906 service manual where they do a test for "runout" with a dial test indicator. Then there is a separate test to measure flatness. These are two tests that are done when replacing the fixed platter. I understand the concept of a dial test indicator and basically what it does... and they show a picture of how to position the guage when checking runout, and how to position it when checking flatness. Two questions come to mind that aren't clear from the manual. I can't tell from the pictures where exactly they place the ball on the fixed media when testing runout. It LOOKS like they put it touching the outside (in the same plane as the media), then rotate once. This would seem to me to really be testing how "circular" the media is. Is that runout? Then, to test flatness, it appears they put the ball on the surface of the fixed media. Surely turning the media with a metal ball on it is pretty bad. So, can someone explain to me the difference between runout and flatness, specifically with regards to a disk platter. And, where they would put the ball exactly for each test? Also, I assume any dial test guage that is graduated correctly will work. However, the HP one apparently has a lever on it for flatness or runout. General dial test indicators do not. Any ideas there? Thanks for helping my confusion :) Jay