  Here, we investigate the role of the interlayer magnetic ordering of CrSBr in
the framework of $\textit{ab initio}$ calculations and by using optical
spectroscopy techniques. These combined studies allow us to unambiguously
determine the nature of the optical transitions. In particular,
photoreflectance measurements, sensitive to the direct transitions, have been
carried out for the first time. We have demonstrated that optically induced
band-to-band transitions visible in optical measurement are remarkably well
assigned to the band structure by the momentum matrix elements and energy
differences for the magnetic ground state (A-AFM). In addition, our study
reveals significant differences in electronic properties for two different
interlayer magnetic phases. When the magnetic ordering of A-AFM to FM is
changed, the crucial modification of the band structure reflected in the
direct-to-indirect band gap transition and the significant splitting of the
conduction bands along the $\Gamma-Z$ direction are obtained. In addition,
Raman measurements demonstrate a splitting between the in-plane modes
$B^2_{2g}$/$B^2_{3g}$, which is temperature dependent and can be assigned to
different interlayer magnetic states, corroborated by the DFT+U study.
Moreover, the $B^2_{2g}$ mode has not been experimentally observed before.
Finally, our results point out the origin of interlayer magnetism, which can be
attributed to electronic rather than structural properties. Our results reveal
a new approach for tuning the optical and electronic properties of van der
Waals magnets by controlling the interlayer magnetic ordering in adjacent
layers.
