You can also use 2 points of a markups fiducial node to specify a circle by copy-pasting a few lines of code to into the Python console as shown in this example.
I’ve created a modified script that allows you to place 2 circles by specifying 6 points on the circumference: Compute bone axis in 3D Slicer by specifying two circles. · GitHub
You can use this by placing one fiducial list containing 6 points then copy-pasting into the Python console:
