Dependence of physical parameters on the adopted ionizing spectrum for the observed systems at z > 2. The marked areas in the upper, middle, and lower panels show the ionization parameter log U, metallicity [Si/H], and relative abundance [Si/C], respectively, versus the parameters of the ionizing SED (Δ₁, Δ₂, Δ₃; from the left to the right) for the best-fit models of the systems at z = 2.3799 (left) and z = 2.8266 (right). The narrower areas indicate the parameters of the models consistent with the observed C iii/C iv column density ratios on a 1σ level.