The deciding factors in determining a Parts' compatibility are the size (S, M, L, XL, XXL) and the generation of hardware (e.g. Gen 1).The generation describes the hardware used to generate long-term compatibility between the Parts. Here is an example:You have a Core in a size Medium belonging to GEN. 1. To upgrade your jacket, you would have to select the desired Parts in the same size (Medium) and within the same generation (GEN.1).