This is just a basic hole gauge tool that goes from 1 to 20 mm. This is not your fancy tool that classifies your screws by thread spacing or measures fillet radius. What this is good for is figuring out what diameter to use when you're trying to create containers for real-world small objects or other things like that. It's also helpful for getting a feel for how easy it'll be to get something in and out of the holder you're designing.

Note that the circles on this gauge are as accurate as your printer - which is the point. The 3 mm circle isn't the best way to check if something is 3 mm but what it will tell you is whether that object will fit in a 3 mm circle as printed on your printer. For it to be useful to you, you should print it with the materials and settings (and printer) that you're actually planning to use. The 3mf is pretty basic PLA settings with .2 mm layer height. The STL is provided separately to make it easier for you to bring it along into your projects.