This is a precision 20mm XYZ calibration cube which improves on the typical calcube. There are two issues with the typical cal cube:
1) Hard to make an accurate Z axis measurement because either you have error from the squish of the first layer or the uneven surface from the raft/support
2) Over/under exposure/extrusion can result in a growth of all dimensions by a small amount which is an offset not a scale error. When you measure two outside edges, this error is additive and is erroneously included in the scale correction.
This cube addresses both of these by making the measurements via the caliper/micrometer depth gauge. By taking a measurement using that instead of the jaws, the overexposure error cancels so that doesn't create error. Also, the Z axis reference surfaces are not touching the bed or any supports.
There is also a 34mm circle cut into the bottom 10mm which can be used to check for XY squareness (using the jaws to compare two opposite corners to the other two).