Modeling of the Mg II doublets originating from the CSM shell (broad features) and the host galaxy’s interstellar medium (narrow features) for DES15S2nr (left) and DES16C3ggu (right). The observed spectra in the 2800 Å region are shown in gray, while the best-fit model and associated 1σ uncertainty range are overplotted in purple. The titles of each panel indicate the CSM shell parameters corresponding to the best-fit model.