The CF flange (originally called ConFlat) is a sexless design — both flanges are identical.   Typical flange materials are austenitic stainless steel types 304L, 316L, 316LN, and surface hardened aluminum.   The seal mechanism is a knife-edge that is machined below the flat surface of the flange.   As the bolts of a flange-pair are tightened, the knife edges make annular grooves on each side of a soft metal gasket, typically a copper gasket.   The extruded metal fills all the machining marks and surface defects in the flange, yielding a leak-tight seal.