  We express the cross section for indirect resonant inelastic X-ray scattering
in terms of an intrinsic dynamic correlation function of the system that is
studied with this technique. The cross section is a linear combination of the
charge response function and the dynamic longitudinal spin density correlation
function. This result is asymptotically exact for both strong and weak local
core-hole potentials. We show that one can change the relative charge and spin
contribution to the inelastic spectral weight by varying the incident photon
energy.
