Spectra of our reference model set at 270 days post explosion compared to SN 2021aefx over the wavelength range 0.35–14 μm (logarithmic scale), with flux in F_ν on a non-linear (arcsinh) scale, as in Fig. 1 of Kwok et al. (2023a). The SN 2021aefx spectrum has been corrected for redshift and extinction (assuming a host-galaxy reddening of 0.097 mag and a MW reddening of 0.008 mag). The synthetic fluxes correspond to the same assumed distance to SN 2021aefx of 18 Mpc; they have not been rescaled or normalised in any way. We include selected line identifications based on their maximum Sobolev EW (see Table 2). The insets show normalised line profiles in velocity space for selected transitions, illustrating variations in line widths and morphology.