An Outline is made up of one or more Outline Parts: Lines, Splines, Arcs, Circles, Polylines, and Fillets. An Outline can be a single part or multiple connected parts (except a Circle, which is always single). See Outline Tools.
When multiple parts are connected, an Outline acts like one object in some respects. For example, when you click anywhere on an Outline, the whole Outline highlights and selects. You can click again on a part to select just the Outline Part (see also Outline Selection).