These are base metal designations used to identify specific alloys for ASME codification, for example SA-335, or Section IX. In this case they are CrMo alloys generally intended for high temperature service. P9 is a 9Cr alloy, P11 is 2 1/4 Cr, alloy and P91 is a 9Cr alloy with creep resistant hardeners such Cb, and N, tossed in. P9 has been a traditional alloy in petrochem applications while P11 and P91 are more generally used in power piping applications.
They are welded with preheat and interpass temperature regimes, can be crack sensetive, and require PWHT.