There were some versions of MKL that didn't fully respect OMP variables, so you had to set the MKL equivalent also. In general, however, they are interchangeable, at least for ATK in the sense that we only use threading (almost only) in MKL. So, in theory, you could thread MKL in one way and other parts of the code in another, but it typically only makes sense in ATK to either assign the same value to both variables (to be on the safe side) or either one - according to the specs MKL should respect the OMP variable.