Model of the rest-frame spectrum of I20210S. Upper panel: full I20210S spectrum (black solid line), along with the local power laws adopted for continuum subtraction (yellow dot-dashed lines), the best-fit reddened narrow (green short-dashed lines) and broad emission profiles (blue dotted lines), and the global spectral model (red solid line) shown superimposed to the data. The intervals used for the local continuum subtraction under the emission lines are marked, along with the masks applied to the [O I]+[Fe X] emission and to the telluric H₂O absorption line (grey bands). Lower panel: zoom on the continuum-subtracted emission lines (black solid line), shown along with the global best fit (red solid line) and the best-fit narrow (green short-dashed lines) and broad components (blue dotted lines). The standardized residuals after the best-fit subtraction are also shown in separate windows below each spectral region. In all panels, the zero-level flux (black long-dashed line) is indicated; in the panel with [O I], the mask applied to the [O I]+[Fe X] emission line is shown superimposed to the data, as well as the one for the H₂O telluric absorption in the panel with Hα+[N II] and [S II] (grey bands).