The D/Focus Systems D/Riser Height Adjuster for D/Focus V3 Gearbox is a set of two different-sized riser and offset blocks that you can use to raise the height of the standard D/Focus V3 Follow Focus gearbox, which is a low-profile gearbox that sometimes can benefit from being raised a bit.
The Tall D/Riser is for use with battery grips in relation to the V3, in cases where the camera lens is much higher than the standard rail height of 85mm. And the Short D/Riser is for Zip style, small diameter lens gears.
You will not need the D/Riser if you are using D/Gears with a standard rail mount or if you have a large diameter lens -- like the Canon 70-200mm -- and are using zip style lens gears.