Appendix 2 contains equations that attempt to determine whether or not a given flange geometry will leak. If the computed rigidity factor is > 1.0, then leakage is predicted.
Appendix 2 calculations are mandatory as of Addenda-2005 and flange designs must satisfy these calculations. These rigidity factor calculations are not mandatory for Pre-1999 Addenda users
Appendix S is non-mandatory appendix and that these calculations are also non-mandatory.