I've updated the documentation here:
http://universesandbox.com/support/newsystems.htm<Orbit> shows the Orbital Elements info.
belt (in <Action>) show how to create asteroid belts.
Although I'll be changing the way belts work in a future release (to make it work with the other orbital element variables and so you can add more than one belt per body).
Look at the code for these systems/simualtions to see for examples of how they are used:
TrES-3 orbiting Star with Protoplanetary Disk
- uses the asteroid belt & orbital elements features
Satellites orbiting Earth
- uses the orbital elements feature
Fomalhaut System
- uses the asteroid belt feature
Please let me know if anyone has suggestions on a better way to do this or needs additional functionality that is missing.