Marking a point M and a point P defines a circle with center M through P. This circle's radius is the distance MP.
After marking the center point M you are asked for the radius in a window.
Marking three points A, B, C defines a circle through these points. If the tree points lie on one straight line, the circle degenerates to this line.
Marking five points produces a conic section through them. If no four of these five points lie on a line, the conic section is defined.