Normalized flux around the UV line C IVλ1550 Å in three post-main-sequence (l-pMS) spectral models with initial masses as indicated in the top left corner of the panels. Line luminosity (computed following Sect. 2.4) is given in the framed boxes in units of log(erg s⁻¹); lg L stands for unclumped (clumping factor D = 1) while lg L_cl for clumped (D = 10) wind, see details in Paper II. When creating the synthetic population here (Sect. 2.3 and Sect. 3.1), we always apply the unclumped models’ predictions (i.e. black line). Other emission lines are presented in Figs. A.1–A.2. Based on this, we predict a ∼40–50 Å-wide emission bump from our population between 1550–1600 Å, which is indeed present in the observations in Fig. 11.