"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your BMW New Six, if you buy parts for your car that are not actually made by BMW, then those are termed "aftermarket". Many people use the term "New Six aftermarket parts" specifically when referring to custom parts for the New Six, like performance parts or custom appearance parts. But even if you just replace your New Six brake pads with OEM-grade factory-style brake pads that are not made by BMW, those too are aftermarket New Six 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".