The difference between the energies of the two set of the d orbitals is called as crystal field splitting energy (CFSE). The degenerate d orbitals split into two levels i.e t_2g and e_g level due to the presence of the ligands. This splitting of the degenerate orbitals due to the ligand is called as crystal field splitting and the energy difference between the two levels is called as crystal field splitting energy.

After the splitting of the degenerate orbitals has taken place the filling of the electrons takes place. Now first 3 electrons goes into into the lower energy three t_2g orbitals. The fourth electron can be filled in two ways:

It can enter the t_2g orbitals causing pairing up of the electron (giving rise to t_2g⁴e_g⁰ electronic configuration) or it can enter into higher energy e_g orbital (giving rise to t_2g³e_g¹ electronic configuration). If the CFSE value of ligand is more than energy required for the pairing up of the electron then it will result in the pairing of electron(2 case) but if the CFSE value of ligand is less than the pairing energy of the electron then it will cause the electron to enter in higher energy e_g orbital.(1 case)