St. Venant's Torsional Constant

Is IX the St. Venant's Torsional Constant or is it the Polar Moment of Inertia?

IX is generally known just as the Torsion Constant. For a circular cross section, the torsion constant equals the Polar moment of inertia. For non-circular cross sections, it usually is less than the polar moment of inertia. If you have the AISC publication T114, you will see it referred to in that document using the expression J. If you have the textbook "Roark's Formulas for Stress & Strain, 6th edition, Warren C.Young, McGraw-Hill", you will find it being referred to using the expression K in section 9.2 of the book.