"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Eagle Summit, if you buy accessories for your car that are not actually made by Eagle, then those are termed "aftermarket". Many people use the term "Summit aftermarket accessories" specifically when referring to custom accessories for the Summit, like performance accessories or custom appearance accessories. But even if you just replace your Summit brake pads with OEM-grade factory-style brake pads that are not made by Eagle, those too are aftermarket Summit accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".