This is a over-under extrusion test gauge.
The theory of it's function is you will print two of the parts, and slide them together. If they are hard to slide together, and the outside warps outward, you are "over extruding".
If the parts are sloppy and loose, when put together, you are "under-extruding".
There 0 clearance between the two parts, so if your printer is on the "not so accurate" side, you might want to accept it a little bit "tight".
When printing, align the seam on the outside somewhere, to prevent it from contacting on the inside/outside of the coupling.
Also, because the "bottom" has no champher, there will likely be an elephant ear. You can use this to help bring your first level into perfection. When you place the two pieces together, place the "top, to the top" and push together. (so the "bottom" of the print, never goes into the slot).
Print it slow. This gauge isn't very forgiving. :)
I really loved the concept of https://www.thingiverse.com/thing:1622868
However I do not like the fact that the "layer lines" are parallel between each other, this allows for over-extrusion to feel good, and proper extrusion to feel "loose", strange, and odd to use, and hard to explain to others, so I designed my own, with a different angle of function.