Photoionisation models of clouds illuminated by a hardened radiation field leaking from H II regions in the [N II] BPT (left), [S II] BPT (middle), and [O I] BPT (right) diagrams. The input spectrum is a 2 Myr SSP, which has been transmitted from a density-bounded H II region. We consider H II regions with a range of hydrogen column densities (N_H, reported in the legend). The legend also shows the escape fraction from the H II region corresponding to each column density value. All models shown have log U₀ = −3.5 and varying metallicity (three representative metallicities are labelled in the left panel to demonstrate that metallicity generally increases along the x axis). Demarcation lines from the literature are shown as dashed (Kewley et al. 2001, 2006), dotted (Kauffmann et al. 2003), and solid black lines (Law et al. 2021).