ASME Section IX allows the contractor to use several different base metals when qualifying the welder. The qualification covers a range of base metals provided the F number of the filler metal is not changed, the positions are not changed, etc.

As for F numbers, consider F6 filler metals, it includes carbon steel as well as austenitic stainless steel. So, the welder can weld carbon steel to carbon steel using a carbon steel filler metal (F6). That same welder can weld austenitic stainless steel to austenitic stainless steel using austenitic stainless steel filler metal (still F6).

What wouldn't be permitted is carbon steel to carbon steel using a nickel filler metal having an assigned F number F4X because that involves a different F number from that used when qualifying.

Best regards - Al