Values of [S II]6716,31/[O III]4959,5007 along the contour of the H II regions labelled in Fig. 8. The solid points and error bars show, respectively, the median and the range in values spanned by the ratio. The cyan line indicates a ratio of 0.5, which we use as fiducial limit for the transition to optically thick gas. Regions for which the range in [S II]/[O III] extends below this limit are classified as featuring optically thin channels (CH); all other regions are classified as ionisation bounded (IB).