Adopted model for the metallicity Z = ρ_Z∕ρ of the envelope. In region 1, the envelope is always saturated and the metallicity Z follows the saturation curve Z_sat (gray), which is set by the saturation vapor pressure. Initially, when the entire envelope is saturated, only this region is present. Upon reaching Phase II a new region (2) develops that is undersaturated (Z < Z_sat). In region 2a, convection ensures that the metallicity Z_cnv is uniform. Region 2b represents a zone where convection does not operate due to lack of energy sources. Here, Z is constant in time for the same mass shell. With increasing accretion of H+He gas, Z_cnv decreases and the region boundaries shift outwards.