Fraction of binary pair-ups occurring within migration traps as a function of the SMBH mass. The vertical axis shows the occurrence f_trap of pair-up events that take place in proximity to predefined migration trap locations, while the horizontal axis shows the logarithm of the central SMBH mass. Different marker fillings indicate the BH generation (filled: 1g; hollow: Ng), while color encodes the viscosity parameter α (orange: 0.01; teal: 0.1; navy blue: 0.4). The two torque formalisms are distinguished by marker shape and line style (squares and solid lines: G24; circles and dashed lines: B16). Trap proximity is defined using a logarithmic tolerance on radial separation (see Section 3.2 for details).