Finally I have found out, that the P-No and Group No definition is given in ASME BPVC Section IX, section QW-420.
Table QW/QB-422 lists all base materials with P-No and Group No. This helps to understand PWHT requirements.
QW-420 BASE METAL GROUPINGS
[i]P‐Numbers are assigned to base metals for the purpose
of reducing the number of welding and brazing procedure
qualifications required.
P‐Numbers are alphanumeric designations: accordingly,
each P‐Number shall be considered a separate P‐Number
(e.g., base metals assigned P‐No. 5A are considered a separate
P‐Number from those assigned P‐No. 5B or P‐No. 5C).
In addition, ferrous base metals have been assigned
Group Numbers creating subsets of P‐Numbers that are
used when WPSs are required to be qualified by impact
testing by other Sections or Codes. These assignments
are based essentially on comparable base metal characteristics,
such as composition, weldability, brazeability, and
mechanical properties, where this can logically be done.
These assignments do not imply that base metals may
be indiscriminately substituted for a base metal that was
used in the qualification test without consideration of
compatibility from the standpoint of metallurgical properties,
postweld heat treatment, design, mechanical properties,
and service requirements.[/i]
