The circle-button constructs a circle if suitable objects are selectd, or shows a popup-menu with all possibilities to construct a circle.
If the circle is not an intersection, the first object determines the midpoint, the second the radius (in case of a point by its distance to the midpoint, in case of a vector by its length), the third object defines the orientation of the circle in space. The third object either is a normal (line, segment, vector) or an object parallel to the circle (plane, triangle, parallelogram or another circle).
To construct a circle with a given length (e. g. 7 l.u.), one has to create a vector of that length first (e. g. by entering ’v=(7,0,0)’ at the command-line) and use this as the radius.