This cover protects the nozzle cleaning pin, which is easy to damage or even deform. It hosts pins of diameter up to 0.4 mm. I designed this for the one that comes with my FLSUN QQ-S Pro.
The tube space inside the cover has two sizes, one section of a smaller diameter for the pin and the other of a larger diameter for the hand of the pin, so that when the pin cleaner is put in, its hand would help secure the entire structure.
I would recommend to use the fine resolution (e.g. 0.6 mm) so that the tube space inside the cover won't go too far from the designed size.