What is the meaning of f6 in gate valves?

F6 Trim means that STEM, BODY seating surface, GATE seating surface, bushing (or a deposited weld), for the backseat and stem hole guide, small internal parts that normally contact the service fluid, excluding the pin that is used to make a stem-to-gate connection (this pin shall be made of an austenitic stainless steel material) are made of a 13Cr material (typical specification type ASTM A276T410 or T420). See API STD 600 - 2009.