The M and Y values are for flange design only and are not meant to be used as gasket seating stress values in actual service. These values are used in formulas in the ASME Boiler and Pressure Vessel Code Division 1, Section VIII, Appendix 2 to give an SM1 or SM2 value based on either gasket seating or the internal pressure.
These values do not take fugitive emissions into account and therefore newer gasket constants, Gb, a, and Gs, based on leakage, were developed to take this into account.