goCFD is recommended when you have very small of the order of the size of the mesh in CFD. What it does it it takes information from CFD in each cell and solves the defined equations describing the process like crystallization, heat and mass transfer, reactions etc.. in gProms and sends it to the CFD again.

In the multi zonal modelling, you specify the zones yourself in gproms. The zones are of the order of 10 to 100 (comparably many times coarser than the CFD mesh). Each zone takes the information from CFD like fluid flow hydrodynamics and solves the model equations describing the process like heat transfer, mass transfer etc. in gPROMS.