Don’t get too excited, it was for limited use on a road.
Categories of vehicle for driving licensing purposes are generally decided according to a vehicle’s weight and/or seating capacity. For example, the driver of a vehicle weighing more than 7.5 tonnes maximum authoried mass would need to hold category C driving entitlement. These categories A,B,C1,C,D etc are recognised throughout the EU. Categorey N is specific to the UK and therefore is not recognised in other EU countries.
Category N entitlement is unique to vehicles that have limited use on public roads and this simple criterion is the basis that determines if the entitlement may be appropriate. Drivers who have held this category before 31 December 1996 may drive any type of vehicle that makes minimal use of public roads. Drivers who do not hold this entitlement (new drivers who passed their test on or after 1 January 1997) would have to hold the appropriate category of entitlement specifically prescribed for the vehicle being driven.
Category N was formally linked to vehicles that were duty exempt, although this has not been the case since the abolition of this taxation class on 1 July 1995.
A vehicle driven under exemption 48 (j) could make any number of journeys as long as each journey did not exceed 1.5 kilometeres on a public road provided the vehicle is :-
a goods vehicle, other that an agricultural motor vehicle, which
is used only for purposes relating to agriculture, horticulture or forestry
is used on public roads only in passing between different areas of land occupied by the same person, and
in passing between any such two areas does not travel a distance exceeding 1.5 kilometres on publlic roads.