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2nd moment of interia is an area moment used on only geomerty (no volume or mass). It is for reduced geometry: cross section of beams and shells. The moment of inertia specified on point mass (MASS21) is a mass moment of intertia with MKS units of kg.m^2. There are real constants Ixx, Iyy, Izz for 3D analysis. These moments are about the element coordinate system axes. The element coordinate system wil be aligned with the global coordinate system if you did not specifically set the orientation. See the help page for MASS21:


The mass moments of intertia are specified with real constants 4-6. KEYOPT (3) must be zero to use the mass moments of interia. for 2D analysis, there are slight differences. See the help.