The upper level energies (the abscissas of the Saha-Boltzmann plot) have been modified by adding to them the ionization energy of the lower ionization stages, that is, the energy scale begins in the ground state of the neutral atom. For the z ions, the correction is shown in the attached figure emphasized by red circle.

where is the ionization energy of species k for isolated systems (eV) and is the correction of this quantity for interactions in the plasma. These modifications have only effect for ion lines (z t 1) and are introduced in equation 1a in order to include the data of the different ionization stages in the Saha Boltzmann plot.

Reference Link:

https://iopscience.iop.org/article/10.1088/1742-6596/59/1/046/pdf