Information of factor_lj for calculation of fpair in harmonic/cut

Dear all,
I am using lammps version lammps-stable_2Aug2023. I am trying to use pair_style harmonic/cut to validate the results of paper:10.1103/PhysRevLett.93.016001.
While trying to under how the pair_style works, I noticed multiplication of “factor_lj” to calculate the fpair in .cpp file.

const double prefactor = factor_lj * delta * k[itype][jtype];
        const double fpair = 2.0 * prefactor / r;

Can anyone help me to understand how this factor_lj is calculated, how it varies with different force fields and other factors it depends on.
Any suggestion is appreciated.

Please see: 4.8.1. Writing new pair styles — LAMMPS documentation