Although most application of HTU and NTU are based on A to B and C, alternative groupings have been used, depending on the driving force for mass transfer and whether the overall basis is the gas, or the liquid, where H and N apply. These grouping are summarized in T. Included are driving forces based on partial pressure, p; mole ratios, X, Y; and concentrations, c; as well as mole fraction, x, y. Also included in T, for later reference, are groupings for unimolecular diffusion when solute concentration is not dilute.