The P-No is a designation by ASME Boiler and Pressure Vessel Code to categorize the chemical composition and weldability of metals used in the fabrication of pressure retaining items. For example, carbon steels are classified as P-No 1 versus low alloy steels that may be P-No 3 or P-No 4. This number provides guidance in the Code for joining metals during fabrication.
This response may be a bit late, hope you see it. P number means "parent" metal. Each P number can have sub-groups, usually differentiating acceptable stress levels. Check this site for a good list of all/most P numbers and some common materials for each.
