Also review blanks - a.k.a. blinds, skillets, etc. per ASME B16.48. This requires similar facing (most that I have seen are flat). B46.1 describes surface texture. B16.20 describes metal gaskets.
my two cents .............in addition to the facings noted above, there are vaiations to the raised & flat face flanges - they can be machined to either a phonographic serrated or concentric serrated finish (standard is a 125-250 AARH). Also available are Tounge & Groove - large or small - and Male & Female - large or small.