This lever can either be an upgrade for pre-2017 X2's or as a replacement for current X2's.
New for 2017, Fox has implemented a new lever in both the Float X2 and DHX2, a 2-position compression switch that effectively offers open and firm settings. The switch affects both high and low speed compression. In the firm setting, the shock will be much stiffer, better for climbing. Naturally, the open setting will be best for descending.
WARNING: FOX products should be serviced by a trained bicycle service technician, in accordance with FOX specifications. If you have any doubt whether or not you can properly service your FOX product, then DO NOT attempt it. Improperly serviced products can fail, causing the rider to lose control
Read more about the installation procedure