Significant correlations: UFO universal plane. Linear combination of N_H and ξ versus v_out. The S23 UFO sub-sample is shown in red circles, T10 in blue diamonds, and C21 in black squares. The intrinsic scatter is 0.23 dex. The solid lines represent the best-fitting linear correlation and the dark and light gray shadowed areas indicate the 68% and 90% confidence bands, respectively.