Example grid slices at 10⁻⁴ Z_⊙ for fixed M_donor = 19.3 M_⊙ (left), 33.9 M_⊙ (middle), and 59.4 M_⊙ (right) for the CO-HMS_RLO grid, where the models start at RLO. The diamonds indicate unstable mass transfer with L₂ overflow (orange) and Ṁ_max (red) separated. Stable mass transfer is indicated with squares with mergers (light blue) and non-mergers (dark blue) within the Hubble time based on the same remnant mass calculation as our default population. The STAR+BH systems evolved in a POSYDON population with ΔM ± 2 around M_donor going into this grid slice are shown as a 2D histogram behind the grid markers, as well as dots for the exact systems. The darker the shading, the higher the density of systems. The black contour contains 100% of the STAR+BH system that produces BBH mergers, showing the overlap between the evolved population and region that leads to BBH mergers in the CO-HMS_RLO grid. The latter can be slightly off from the exact location of progenitor properties due to underlying histogram binning, but also due to the 4-dimensional nature of the grids. It provides an indicator of the parameter space region of the BBH mergers progenitors.