The order in which atomic orbitals are filled is based on what factor?

The highest energy level available is based on the factor

Aufbau means "building-up." Aufbau principles determine the order in which atomic orbitals are filled as the atomic number is increased. For the hydrogen atom, the order of increasing orbital energy is given by 1 s < 2 s = 2 p < 3 s = 3 p = 3 d, etc. The dependence of energy on nalone leads to extensive degeneracy, which is however removed for orbitals in many-electron atoms. Thus 2s lies below 2 p, as already observed in helium.