To determine the distance between two postal codes (as a car over the road, or in a straight line), enter two postal codes.
With the exception of the webshop subscription, all subscription types have access to the straight line distance. However, the over-the-road algorithm uses the travel time table (shortest route) which requires a 'specials' subscription.
The distance between 6PP zip codes works up to a 5 km limit. When the distance is higher, fall back using 4PP zip codes.