# What is meant by the highest occupied energy level in an atom?

Ordering orbitals by energy is straightforward; energy is quantized, so the higher the principal quantum number $n$, usually the higher the energy of the orbital.
Note that the $4 s$ orbital can be lower in energy than the $3 d$ sometimes, but it is actually the valence orbital for most first-row transition metals and is actually higher in energy in those cases, so this diagram is not entirely correct.