Distribution of the reference synthetic spectra in the atmospheric parameter space (T_eff, log(g), [M/H] and, [α/Fe] are in K, cm/s², dex, and dex, respectively). The cool-star and hot-star grids are shown with open circles and filled triangles, respectively. Any combination of the shown stellar parameters has been considered when building these grids, except for the hot-star grid in which no variations in [α/Fe] have been considered (see filled triangles in the right panel).