This is a calibration set that includes three circles, and is meant to be used to calibrate as many settings as possible at once.

One circle is solid to replace a calibration cube (print at 100% infill), one has only a thin wall, and one has a slightly thicker wall with a bridge covering the top. All three are printed at once, which lets you to test your reversal settings and also gives the parts time to cool while the others are printing.

I've been using this set for a while (every time I need to recalibrate for some reason), and it seems to test more characteristics faster than any other set I've tried.