Each ring is divided into districts of about 45 degrees, and there eight districts per ring. The rings are large enough it is impossible to notice their curvature except in large, open areas. It has a population of several million, but a more precise number is difficult to calculate.
Hub: Warehouses, industrial operations, and ship maintenance firms.
1st Ring: Hotels, casinos, and shopping. There are also industrial areas, including a area owned by the Winged Hussars human (Mercenary|mercenary)) company that includes warehouses, maintenance shops, and a training area. It is also the home of Peepo's Pit.
2nd Ring: Commercial office space and permanent housing. The Karma offices of the Winged Hussars are in District Four.
3rd Ring: Mostly offices.
From the Books: "Rick had to admire Karma Station as the Coronado braked with her ion drive. Compared to all the stations and habitats he’d seen in his home star system, it was tremendous. Three huge rings rotating around a stable central zero-gravity hub. Dozens of starships were parked around the station, many of them warships, with some freighters and a few ships of unknown utility. The planet looked a bit like Earth with huge oceans, white icecaps, brown/green land masses, and swirling cotton candy clouds. It was the most interesting place he’d seen since leaving home." - Winged Hussars, p. 89.