  Here we systematically investigate the impact of the spin direction on the
electronic and optical properties of transition metal phosphorus
trichalcogenides (MPX$_3$, M=Mn, Ni, Fe; X=S, Se) exhibiting various
antiferromagnetic arrangement within the 2D limit. Our analysis based on the
density functional theory and versatile formalism of Bethe-Salpeter equation
reveals larger exciton binding energies for MPS$_3$ (up to 1.1 eV in air) than
MPSe$_3$(up to 0.8 eV in air), exceeding the values of transition metal
dichalcogenides (TMDs). For the (Mn,Fe)PX$_3$ we determine the optically active
band edge transitions, revealing that they are sensitive to in-plane magnetic
order, irrespective of the type of chalcogen atom. We predict the anistropic
effective masses and the type of linear polarization as an important
fingerprints for sensing the type of magnetic AFM arrangements. Furthermore, we
identify the spin-orientation-dependent features such as the valley splitting,
the effective mass of holes, and the exciton binding energy. In particular, we
demonstrate that for MnPX$_3$ (X=S, Se) a pair of non equivalent K+ and K-
points exists yielding the valley splittings that strongly depend on the
direction of AFM aligned spins. Notably, for the out-of-plane direction of
spins, two distinct peaks are expected to be visible below the absorption
onset, whereas one peak should emerge for the in-plane configuration of spins.
These spin-dependent features provide an insight into spin flop transitions of
2D materials. Finally, we propose a strategy how the spin valley polarization
can be realized in 2D AFM within honeycomb lattice.
