In fact, circles and circular arcs are ellipses and elliptic arcs. When being created, they are set to be the circular special cases; but, they do not differ from elliptic objects in anything more significant then just a few property values. Therefore, circles have two radii and other properties which might seem useless.
See Elliptic arcs for full description.
•  Circular arc defined by three points on its perimeter
 Circular arc defined by three points on its perimeter
   
Creates a circular arc defined by three points on its perimeter. The three points must not be collinear.
•  Circular arc defined by the center point and radius
 Circular arc defined by the center point and radius
   
Creates a circular arc defined by the center point and radius. The radius, the end point parameters, etc. can be modified in the Property editor.
•  Circle defined by three points on its perimeter
 Circle defined by three points on its perimeter
   
Creates a circle defined by three points on its perimeter. The three points must not be collinear.
•  Circle by the center point and a point
 Circle by the center point and a point
   
Creates a circle defined by the center point and a point on its perimeter.
•  Circle by the center point and radius
 Circle by the center point and radius
   
Creates a circle defined by the center point and radius. The radius can be modified in the Property editor.