What is crystal field splitting energy? How does the magnitude of decide the actual configuration of d orbitals in a coordination entity?

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 t2g and eg 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  t2g orbitals. The fourth electron can be filled in two ways: It can enter the t2g orbitals causing pairing up of the electron (giving rise to t2g4eg0 electronic configuration) or it can enter into higher energy eg orbital (giving rise to t2g3eg1 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 eg orbital.(1 case)