A number is a weld metal chemical composition, it can be obtained from procedure qualification coupon. It is only apply to ferrous metals in ASME IX Table QW-422 or QW-404.5.
A number may be determined from chemical analysis of weld deposit or MTC’s electrodes.
As mentioned in above, the A number like P number and F number is an essential variable in most welding processes. It means if you have a WPS and you need to use that in other job with different A number (it means you are using different filler metal) then you need to re-qualify your WPS because it is essential variable. The A number 1 and 2 are exceptions. You can consider it as nonessential variables/