ASTM A403/A403M-04 Standard Specification for Wrought Austenitic Stainless Steel Piping Fittings
ASTM A182/A182M-02 Standard Specification for Forged or Rolled Alloy-Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service
The F indicates a forged product while WP is for a wrought (which could be seamless or welded).
You should purchase both specifications and review them carefully.