Membership probability against angular distance to the center of NGC 6231 for all members identified in this cluster (log-scale). The vertical dashed line separates the two subsamples, one closer and one farther than 2 pc from the cluster’s center. The horizontal dashed line indicates the probability threshold, p₀, below which the KS test rules out the probability distributions of both subsamples being drawn from the same parent distribution. The resulting high- and low-probability members are indicated with dark green and orange, respectively.