This Standard covers flanges (similar to those covered in ASME B16.5) that have orifice pressure differential connections. Coverage is limited to the following: (a) welding neck flanges Classes 300, 400, 600, 900, 1500, and 2500; (b) slip-on and threaded Class 300; (c) welding neck flanges Class 400 in U.S. Customary units in Mandatory Appendix II.