Recently, the transition point between the Harrison Type-A to Type-B kinetics regimes as well as the emerging intermediate AB transition regime have been analysed in detail by making use of Lattice Monte Carlo simulations of tracer depth concentration profiles as a function of diffusion time and distance between grain boundaries e.g. [7-9]. The principal model in this area has been the grain boundary slab model. However, depending on the diffusion time and grain size a given tracer atom can be expected to cross a number of grain boundaries in its diffusion time. This has recently been taken into account in square and cubic grain models for determining the limit of the Harrison Type-A regime . In the present study, we determine the limits of the intermediate AB transition regime where we have used the Lattice Monte Carlo method to analyse the tails of tracer concentration depth profiles. The applicability of different solutions for the grain boundary diffusion analysis is numerically investigated for the 3D model. The solutions are those by Whipple-Le Claire [12,13] and Suzuoka , Bokstein, Magidson and Svetlov , Levine and MacCallum  for the type B kinetic regime and that by Divinski and Larikov  for the intermediate Type-AB regime. We also review the progress that has been made concerning the limits of the various Harrison regimes supplemented with the results of the present simulations. An empirical factor of 1.5 should be applied when Suzuoka or Whipple-Le Clair solutions are applied to the analysis of the tracer diffusion profile, tail section, in the polycrystalline material and an empirical factor of 2.0 should be applied when Bokshtein-Magidson-Svetlov solution is applied to the analysis of the tracer diffusion problem in the polycrystalline material.