According to User's Guide - Input overview based on analysis category - Modal - Specifying the loads,
"Extra mass which may have been ignored as insignificant in the static model (e.g. a flange pair) can be directly entered here."

However, I believe that a rigid weight, such as a flange pair between two supports, will affect to the system's modes of vibration significantly.
Why is it possible to delete masses from mass model? Is this to ignore the redundant system's modes?

I think, if piping system is complicated and it's difficult to identify the system's mode of vibration at the specified portion,
piping model should be simplified by deleting redundant members or far members from the subject portion.

Could you please advise how to make appropriate mass model of modal analysis in a general way?

Best regards.