Left panel: Distribution of flux residuals (observed – modelled) with spectral wavelength for 159 591 reference spectra. Each row presents one spectrum and the spectra are sorted by their S/N. The dashed blue and red lines indicate the position of S/N equalling 50 and 100, respectively. Some typical stellar lines within the RVS spectral region determined by Contursi et al. (2021) are indicated as well. The lower panel is a zoom-in plot of the upper one to show the distribution of the residuals in the DIB window (860–868 nm). Right panel: Four examples of the reference spectra (black lines with observed flux errors) and their derived stellar templates (red lines). The orange lines are the flux residuals (observed – modelled) with dashed black lines indicating ±5% of the continuum.