It determines the probability of an electron occupying a state at an energy level E. This takes into account that a collection of electron must obey the Pauli Exclusion Principle. Now we consider the interaction for which no two electrons can be in the same quantum state, which is essentially obedient to the Pauli Exclusion Principle. We assume that we can have only one electron in particular quantum state ψ including spin associated with the energy value E. We therefore need those states that have energies E3 and E4 to be not occupied. Let F (E) be the probability that an electron in such state, with new energy E in this new interaction environment.
F (E1) F (E2) [1- F (E3)] [1-F(E4)]
The square brackets represent the probability that the states with the energies E3 and E4 interacting to transfer to E1 and E2 has the forward process.