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