Mass ratio q = M_{BH, L}/M_{BH, H} of the lighter-to-heavier BH and effective spin parameter χ_eff for all BBHs produced in the 96 sets simulated in this work (Section 2). Colored thick lines indicate the analytic relations (Equation (7)) that delimit the allowed parameter space for maximally spinning (χ₂ = 1) second-born BH, for spins aligned (cos θ₂ = 1) or anti-aligned (cos θ₂ = −1) with the binary orbital angular momentum. Black thin lines show the analytic relation expected for low spinning (χ₂ ∼ 0.1–0.5) and aligned (cos θ₂ = 1) second-born BH. The analytic relations assume that the second-born BH is the heaviest one (M_{BH, 2} = M_{BH, H}), unless different specifications.