Hα/[N II] versus Hα/[S II] diagnostic diagram. Based on the discussion in Stasińska et al. (2006), simple separation lines are plotted to indicate regions with different ionization mechanisms: the bottom left quadrant is occupied by photoinized objects, while the upper right quadrant is occupied by shock-heated objects. MAPPINGS III (Allen et al. 2008) radiative shock models for solar abundances and preshock electron density n_e = 10 cm⁻³ are overlaid.