Best-fitting models for eight representative spectra in the vicinity of the Na ID complex. The original spectra are shown in black and the pPXF best-fit models in orange; the red profiles represent the best-fit models obtained with the simultaneous multi-component approach. Green, grey, and blue profiles are obtained from Eq. (1) and represent the different kinematic components used to model the Na ID absorption; in the last four panels, the Na ID emission line contribution (modelled with Gaussian profiles) is shown with an arbitrary offset in the y-axis.