What is crystal field splitting energy? How does the magnitude of $\Delta_{o}$ decide the actual configuration of $d$-orbitals in a coordination entity?

(N/A) The degenerate $d$-orbitals (in a spherical field environment) split into two levels,i.e.,$e_{g}$ and $t_{2g}$,in the presence of ligands. The splitting of the degenerate levels due to the presence of ligands is called the crystal-field splitting,while the energy difference between the two levels ($e_{g}$ and $t_{2g}$) is called the crystal-field splitting energy. It is denoted by $\Delta_{o}$.
After the orbitals have split,the filling of the electrons takes place. After $1$ electron (each) has been filled in the three $t_{2g}$ orbitals,the filling of the fourth electron takes place in two ways. It can enter the $e_{g}$ orbital (giving rise to $t_{2g}^{3} e_{g}^{1}$ electronic configuration) or the pairing of the electrons can take place in the $t_{2g}$ orbitals (giving rise to $t_{2g}^{4} e_{g}^{0}$ electronic configuration). If the $\Delta_{o}$ value of a ligand is less than the pairing energy $(P)$,then the electrons enter the $e_{g}$ orbital. On the other hand,if the $\Delta_{o}$ value of a ligand is more than the pairing energy $(P)$,then the electrons enter the $t_{2g}$ orbital.