Reloadable composite engines are essentially the same as single use composite engines. They use the same fuel, timing charge and ejection charge but are assembled in an aluminum case. The case is reusable. After it has been cleaned, it can be reassembled with a new nozzle, fuel, delay charge and ejection charge, and used again.
The chief benefit of reloadable engines is that they are less expensive than single use composite engines. And, it is fun to build your own engine before a launch.
High power model rocket engines are only available as reloadable engines