i am doing flange leakage using ASME method in Caesar II version 5.1. WHen we enter forces and moments in last sheet in that why optional is written with forces and moments even when forces and moments affect our result greatly

If I understand your question, it is optional because you can choose to insert the external forces and moments in these fields or you can convert them in pressure, add to design pressure and insert in the pressure field. B31.1 ask for the second method.

For b31.1 read para. II-4.2.3 Analysis of Flange