Justification of the [S II]λ6731/16 cutoff. (Top) Histogram of the [S II]λ6731/16 for background-subtracted regions (gray markers with error bars), with the best-fit Gaussian model (solid blue line) and vertical lines indicating the location of −1σ (dotted line), −2σ (dash-dotted), and −3σ (dashed) plotted for convenience. The uncertainties are propagated from the points in each bin. (Bottom) Same as top panel, but for DIG regions.