Mean (relative) differences between the main ionization fractions as predicted by our approximate NLTE description following Puls et al. (2005), and our current “exact solution” (complete NLTE, detailed CMF transport), as a function of τ_Ross ≤ 1 and v (in km s ⁻¹). Displayed are the models with the largest differences (s6a) and the smallest ones (d6v) within our grid. The mean (see Eqs. (6) and (7)) refers to 11 selected elements from C to Ni, and its 1-σ deviation is displayed in gray. The vertical lines indicate the location of the sonic point.