Outline Circles are closed circle shapes. An Outline Circle is defined by three points – click the first point on the circle edge (pick surface if needed), then one further along on the circle edge (think opposite point), then move the mouse to define the diameter of the circle so that it matches the shape being traced and click to finish the Circle. Circles cannot be joined with other Outlines.
A circle can also use the Radius Control drop down on the ribbon toolbar to define one with a preset radius, or via its Properties. See also Radius Library.