The second momentums are given on polar coordinates for the cross section. They are given relative for the center of gravity, not for the origin of the coordinate system.
The second momentum in axial direction Iaa and in radial direction Irr are usually available. For the tangent direction the torsional moment should be given, but since this is not usually known the polar moment Itt = Iaa + Irr can be used as replacement. This is only exact for a circular cross section, but the torsion load is not the main loading to the outer ring, so it could be acceptable.
The deviatory momentum is defined as . Depending on the literature also definitions with a negative sign are usual. The deviatory momentum is zero for symmetric cross sections.