Four parts; Plenum+Lid, and Cartridge+Lid.
Choose between regular and XL cartridge. XL is just 10mm longer, which can make a big difference in air flow.
Choose XL for max carbon content and filter duration, and if you dont mind the fan (200Pa+) has to run faster/noisier. You probably have to use pellets instead of granular carbon too.
Choose regular if you have granular carbon and/or a wekaer fan, and/or wish to have a more silent experience. Regular also allows for somewhat higher airflow, which could make a small difference compared to XL in chamber heat up times.