The UAX13 was designed to cater for exchanges where the ultimate requirements would not exceed 200 lines. They were an updated version of the UAX6 in as much as they catered for 200 lines, three digit numbering and 2-10 PBX hunting. Unlike the U6, the U13 allowed junctions to automatic exchanges (as well as the parent manual board), was the first UAX to allow tandem working to other UAX's, and used group selectors, instead of digit absorbing on the finals.
The UAX 13 is probably the best known of all the the GPO range, as it became the the last type of UAX to remain in the network before the digital era. As it is designed around racks that are a very convienant size it is also the most preserved UAX type.