Eccentric sleeves are used for u drill to increase size in 0.2 mm and reduce it too like this so you can achieve the target size you need. 

20-25 means 20 mm shank of u drill 25 mm of holder inner diameter

20-32 means 20 mm shank of u drill 32 mm of holder inner diameter

25-32 means 25 mm shank of u drill 32 mm of holder inner diameter

25- 40 means 25 mm shank of u drill 40 mm of holder inner diameter

32 -40 means 32 mm shank of u drill 40 mm of holder inner diameter

ID STAND FOR INNER DIAMTER OF SLEEVE

OD STAND FOR OUTSIDE DIAMTER OF SLEEVE

Using the eccentric sleeves, you can vary and adjust the diameter of the hole by /- 0.2 mm with ease