The moments of intertia (Ixx, Iyy etc.) have to be define, when using the *PART_INERTIA keyword card. Furthermore, the code must know the appropriate reference axes. Sometimes the inertia values are provided to the analyst in a coordinate system that is different than the global coordinate system. In such a case, IRCS=1 allows the user to input those inertia values directly without firstly convert them to the global system. LS-DYNA will do the conversion automatically. Whether a global or local reference system is used in the input, that reference system always has its origin at the center-of-mass of the rigid body.