I am pretty sure that you are looking at "Average Roughness Height". Not sure what the second "a" stands for. The measurement is always in microinches. You are looking at 250 X 10 -6 inches, and so forth. The measurement is from the peak to valley of the bumps making up the...
I have a book that used to be given out free (or cheap, I can't remember) by Lincoln Welding. The whole book was about general ideas on how to use weldments for all kinds of machinery bases and parts. It just gave general ideas on how to bend and weld. It does not help with strength...