The world of shotshell powders is based upon 2F black powder. I'm sure you know the amount of powder used in shotshells back then was referred to in drams and the same 'drams equivalent' has been carried forward to todays boxes of modern shotshell.

Similarly, black powder measures are also calibrated with 2F as the standard...ie: a 100grn powder measure setting holds 100grns volume of 2F which also weighs 100grns weight.

So 2F granulation of black powder volume and weight are the baseline standard in the world of shotshell powders.