Basically they are parts that are parts that are not available on all cars.
For example, the racing flywheel, triple plate clutch, stage 4 and 5 turbos, stage 4 and 5 weight reduction, higher powered brakes, and even tires.
These parts are usually applied on race cars, RMs, and other cars by default. Many of these parts are actually in the game, it is just the game automatically gives certain cars these parts by default.
Like the 787B Race Car for instance, it has the racing catalytic converter, racing flywheel, triple plate clutch by default. When you obtain these parts (through hybriding programs), you are able to put many of these parts on just about any car.
I use the triple plate clutch a lot, mainly on certain cars with hard shifting transmissions so that I do not get a miss shift while shifting fast on my G27.
Tires for example. Pretty much every type of tire has a Type V and Type C tires (you get them through hybriding programs). Type V is inline grip and Type C is horizontal grip.
Through testing hours on end, I have found certain cars that I have figured out what type of tire they have by default. Some of which have Type C, Type V, and even mixed matched types of tires by default. But I can chose which ones I want to put on the car.
Some interesting things I have discovered through testing these parts. I don't use them to get any advantage, I just like to have a different feel of the car. You should try some of them.