These are the energies of valence atomic orbitals in the elements. They were found using Photoelectron Spectroscopy, which measures the energy needed to ionize electrons from atoms in the gas phase.
Usually we use these to make MO diagrams. They tell us the relative energies of the AOs that we start with. You can also look at the periodic trends, etc. The lowest value is the same as the first ionization energy.
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