Many of the routes in North Wales - especially the Cambrian lines - had restrictions on the weight of locomotives allowed to traverse them. These restrictions were summarised in the "Route Availability" documentation.
In this system, each class of locomotive is allocated into a route availability group (between 1 and 10), based primarily on the weight per axle. Each route is similarly allocated to a group between 1 and 10. The rule is that a locomotive may not work over a route that has a route availability (RA) group lower than its own RA group.
e.g. A loco with RA6 cannot work over a RA5 route, but can work over an RA6 or RA7 route.
There are exceptions to these rules - both additional restrictions and additional authorities - and these can be seen below.
The information on these pages comes from the BR LMR Route Availability Guide, and the Freight Train Loads Book, both issued in 1969.