Debris is the surface layer of a body that broke apart, left on the broken body's orbit. It is made by Collision. Ships and stations that were at the broken body move to its debris, and so do any moons of the broken body. Worldgen also places debris in some systems: among the fragments of a Shattered system, and as the ring around a Ringed Giant.
Debris keeps its kind whatever it is made of: the kind rules skip it, as the kind records where it came from.