Little is known about the Rim planets (the information is compiled from various sources: long range probes, Capellan Free Merchants, etc.).
The Rim is a loose confederation of planets with a small spacefleet called the Flight. The government is known as the Coalition. The Rim, like the UPF, has been invaded by the Sathar in the past, leading the Capellan Free Merchants to search for allies. For this reason the Free Merchants ventured into Frontier space shortly after the end of the First Sathar War. The Coalition and the UPF signed multiple treaties and mutual protection pacts against alien invaders.
The UPF and Rim enjoy cordial relations and the two civilizations complement each other; the Frontier provides military experts, industrial savvy, and organizational workings while the Rim contributes vast planetary fleets of explorers and merchantmen, its venturous races, and its vulnerable yet strategic location in the path of Sathar invasions.
It must be remembered that the Rim planets are not suburbs of the UPF and that the Capellan Free Merchants, while keeping a low profile, can be the most devious of mega-corps.