Outer rim thickness is 2mm, the central boss hole  is 4.00mm and accomodates bushes allowing both 2.00 mm and 3.00mm axles to be fitted to the wheel.

In other words axles of 2, 3, and 4mm can be used with the wheels.

The boss area is sufficiently thick to allow boring out to other sizes, or for some purposes a small bearing may be fitted.