Top: emissivity grid for the [SIII]6312/9069 line ratio, from PYNEB. For densities n_e below the critical value log n_e ⪅ 3, the temperature is almost independent from the density. Centre and bottom: dependence of the [SIII]6312/9069 line ratio on the electron temperature and of the [SII]6716/6731 line ratio on the electron density, for a fixed value of the second quantity. Shaded areas indicate the range of observed values for the HII regions (purple) and DIG (green). The coloured area spans the first to third quartile of the distribution; the median is indicated as a dotted line. The method is unable to recover densities below 1 cm⁻³ (corresponding to a line ratio [SII]6716/6731 ≲ 1.45).