I think the point they're trying to convey is that no two codes resolve to overlapping rectangles. In my town there are two zip codes, but both of these are ha

I think the point they're trying to convey is that no two codes resolve to overlapping rectangles. In my town there are two zip codes, but both of these are handled by the same post office, so it doesn't really matter which one you use. This can cause confusion though. Ensuring that multiple codes don't resolve to the same place (i.e. rectangle) means that there isn't this confusion.