HFSS initially generate the mesh, then calculate the S-parameters, then refine mesh, calculate S-parameter again, compare the S-parameter difference, if the difference satisfy the stop condition which you set by deta-S, then HFSS treat the mesh is ok, and do the calculation and sweep with this mesh file, if the difference dont satisfy the stop condition, it will do further mesh refinement, except the number of iterations exceed the maxium iteration rounds you specified, in this case, HFSS will use the mesh file got from the last round to proceed the calculation.
Best Regards,