Impact parameter b_impact of identified foreground halos with respect to their virial radius, r₂₀₀, along each FRB sight line, plotted as a function of the impact parameter. The colour map illustrates the mean mass of the halos, whereas the associated uncertainty is propagated into the error bars on their corresponding r₂₀₀. The dashed line illustrates the maximum distance, b_impact = r₂₀₀, at which a foreground halo can be intersected by the FRB sight line and potentially contribute to RM_obs if a halo extends to 1 virial radius, r₂₀₀. In addition, we also show the case where the halos extend to two virial radii, 2 × r₂₀₀, with the dash-dotted lines.