I believe that is the limit on damage objects and yes, they do have to be Debris0 - Debris11. These directly correspond with Part0 - Part11 in the damage.ini file. The data in the Part(n) section dictates what force will cause the part to detach and how much weight the car loses when the part detaches. The Debris(n) part is used in the gen file to tie an object back into the parts section of the damage.ini. The naming is hard coded.
You can also use the frontwing and rearwing instance names, giving you a couple more bits of debris. When these get removed, any aero properties in the hdv that are associated with the wing will be lost. So if you are getting down force from a rear wing and it gets detached, the car loses that downforce.
I think the radcover instance is a hold-over from an older ISI game. It was supposed to be an object that could be removed and it would cause the engine to have overheating problems. However, there is nowhere I see where values for location, cooling amount, force to remove, etc can be set. That's why I'm not certain if it is used.
Lastly, you will find in the default damage.ini file that certain Debris(n) instances are associated with suspension in the damage.ini. If you leave that code in place, and use the same Debris(n) instances for your parts, they will move up and down like suspension parts. So, you will want to remove the code that associates the parts with the suspension if you want to use the debris instances for bodywork.
Tah Tah For Now!
"I can do all things through Christ who strengthens me." Phil 4:13