The domain may be further sub-divided into regions; each of which is similarly bounded by six coordinates: two latitudes, two longitudes, and two depths. Within each region, resolution can be specified as constant or non-uniform along each of the coordinate directions. Although non-uniform resolution is permitted, it is not allowed in the sense of generalized curvilinear coordinates. Resolution along any coordinate is constrained to be a function of position along that coordinate. For instance, vertical thickness of grid cells may vary with depth but not with longitude or latitude.