Emission line ratios log([OIII] λ5007/Hβ), log([SII] λλ6717,6731/Hα), and log([NII] λ6584/Hα) versus log(Σ(Hα)). The vertical line indicates the threshold distinguishing DIG-dominated regions, defined as areas with Σ(Hα) < 10³⁹ erg s⁻¹ kpc⁻². The black data points correspond to the mean values within the different Σ(Hα) bins defined in Sect. 3.2. There is a clear increasing pattern of [SII]/Hα and [NII]/Hα ratios and a decreasing trend of [OIII]/Hβ as Σ(Hα) decreases. The colour scale at the right of each panel indicates the EW(Hα).