Depending of the welding standard, base metals are grouped together to limit the number of qualification tests required to meet production requirements. AWS D1.1 groups them together based on carbon equivalency and similar "weldability". ASME groups the base metals by P numbers, AWS B2.1 groups them by their M number, and NAVSEA by their S number. Generally, the materials in each group, i.e., P, m, or S numbers, have similar chemistry and mechanical properties (similar, but not necessarily the same).
Best regards - Al