That's easy to explain. The handpiece of the rotary tool is standard with a chuck for 2,35 drills.
You can change that chuck to 3 or 3,175 mm. But that is not easy and quick to do. So I have ordered 2 more handpieces and ordered also the extra chucks of 3 and 3,175 mm. Now I have 3 handpieces with 3 different chucks and I can use these for the most common measurements of small cutters.
Do this only if you have these cutters in you toolbox. If you don't have 3 mm. in your toolbox, don't buy it. The 3,175 mm. is handy, most small drill bits have this measurement even like 2,35 mm. The handpiece of the rotary set is identified with the number 202, if you need spare parts for it. Everything is available for it on AliExpress.