Radius

If enabled, the radius will be set as the specified value. If disabled, the radius will be set at the distance between the first two points selected.

Keep Center or Start/End

The Keep Center option will take the first point selected as the center, the second point as the first arc beginning, and calculate the arc end from the vector defined by the first point and third point. The radius used will be determined by the Radius parameter.

The Keep Start/End option will use the first and second points selected as the arc ends and calculate the center using the Radius parameter if specified, or else the average distance between the first and second points, and the first and third points. The plane will be defined by the three selected points.

Points

Select 3 points.