Asteroid belt, the asteroid clusters all orbit the star in that ring, other asteroids are too thinly spread out to register on the FSS as clusters. In a point of interest, stars with very large asteroid belts can't be jumped directly to using the fleet carrier, you can jump in with a regular ship and then map the bodies if it has any, then the fleet carrier can jump to one of them, but it can never jump to the primary star.
For instance HD 144719 can't be jumped directly to using a carrier, I had to use Jumponium to get there and map the system then jump the carrier in. Attempting to jump to the primary star just gave me the message "no free slots", the radius of the asteroid belt is just under 2,000ls;
The Galactic Positioning System of Elite: Dangerous at your service.
www.edsm.net