15.2.3—That the Boltzmann constant k is given by k = R / NA

Boltzmann constant k=R/N_A, so kT is the thermal energy scale per particle while RT is the corresponding molar scale.

Use k when counting individual particles and R when working with moles; the two forms describe the same thermal physics at different scales.

The average translational energy scale of one molecule uses kT, whereas one mole uses RT.

k and R are not interchangeable numbers; their units and particle-count basis differ.