The latest production changes for the aluminum rear uprights are two new roll center positions, giving a total of 4 different roll center positions. The super-fine tuning roll center positions allow for very small set-up adjustments to tailor your car to different track conditions, and also depending on tire choice (rubber or foam).
Roll center positions may be additionally adjusted by using shims of different thickness to alter the height of the roll center link. Please refer to the XRAY Set-up Book for more information about the effects of roll center adjustment and how to make the adjustments.
Source : XRAY