The first moon of the fifth planet of the secondary of the Nivenia System, B5.1 is the only non-ecumenopolis planet in the Nivenian Empire that classifies as a full province. However, is was settled relatively recently, so it has a newer architecture than, say, A11, or even A6.1. B5.1 is unique as it possesses large quantities of easily accessible palladium and platinum on the surface. These catalysts are essential for the Nivenian Empire's chemical manufacturing purposes to build ringworlds and the like from just atmospheric methane and ammonia in gas giants.
B5.1 is tidally heated, and colonies on the surface take advantage of this as an energy supply. The colonies and cities themselves are giant O'Neill cylinders distorted into circular paraboloids to simulate higher gravity. These are suspended in vacuum chambers to prevent atmospheric drag.
B5.1 is also an administrative hub for the entire secondary subsystem of the Nivenia System, which gives it unique leverage to the Nivenian Empire as signals take a day or two to arrive. These duties are shared with B4, as it was colonized first, but govern both B3 and the Nivenia B ringworld coalition.