P Numbers are an ASME number system, as are Group numbers. Steels and Steel Alloys are included within P or S# 1 - P or S# 11. Note the new term, "S" Number... S numbers are "basically" P Number materials but may not be approved for use in all ASME Codes. Group numbers, in the most basic terms, differentiate tensile strengths and chemistry. Reference ASME IX, QW-420 for a better definition than I'm able to provide here.
In a nutshell; P-Numbers are grouped into alloy "families."
Hope this helps?