The energy required to remove an electron from an atom in the gaseous state is called its first ionization energy. For an atom containing more than one electron, more than one electron, even all the electrons can be remove one by one. In that case, the ionization energies are called successive ionization energies.
The first ionization energy in a multi electron system corresponds to removal of the most loosely bound electron, which is the electron occupying nth orbital. Thus it is the energy of the outermost level, i.e. `E_n` .
Successive ionization energies would give an idea of successively interior energy levels.
However, this relationship holds good, only when there is a single electron in the nth orbit. Otherwise, a more precise idea about the internal structure of that particular atom is necessary for such qualitative correlation.
For example, for potassium ion K(19: 1s2,2s2,2p6,3s2,3p6,4s1), the first ionization energy corresponds to the energy of 4s orbital. The second ionization energy corresponds to the energy of 3p orbital, the 8th corresponds to 3s level, the 10th corresponds to 2p level and so on.