I am sorry I can only give you a general answer to your question because the specifics are beyond my area of expertise. A335 is an ASTM standard specification for Seamless Ferritic Alloy-Steel pipe for High Temperature Service. According to the standard, "Selection will depend upon design, service conditions, mechanical properties, and high-temperature characteristics." There are chemical compositional differences between different grades that affect their physical properties (e.g., strength) and corrosion resistance. These physical property differences come into play when doing detailed design calculations. For example, according to ANSI/API 530 (Calculation of Heater Tube Thickness in Petroleum Refineries),  A335 grade P11 has a limiting design metal temperature of 595 degrees C while P21 and P22 of 650 degrees C. So selection of potential grade(s) for a specific application will be partly determined by the output of the design calculations.