Danabasoglu, G., and J. Marshall, 2007: Effects of vertical variations of thickness diffusivity in an ocean general circulation model. Ocean Modelling, 18, 122-141, doi:10.1016/j.ocemod.2007.03.006.
This paper may help answer you question. I am trying to diagnose the horizontal tracer diffusion term, which you can find in the 3.3.2 of The POP Reference Manual. In my personal view, the last two diffusivities are used in the horizontal diffusion options of the Gent-McWilliams parameterization (7.1.3 of the POP Reference Manual). But I don't know whether and how the HOR_DIFF should be considered when diagnose the total horizontal diffusion term.