Here's something Al posted a few years ago...
https://app.aws.org/forum/topic_show.pl?tid=24271"A numbers are groupings for the ferrous metal weld deposits. In certain circumstances you can use the A number determined by the filler metal manufacturer. In other cases, such as when welding dissimilar metals, you have to send a section of the weld deposit out for chemical analysis and then see what A number it matches."