An open or closed element with vertices or endpoints.
For a profile consisting of lines and arcs, the elements may or may not be tangent at their endpoints.
For a closed b-spline curve, a closed curve that is not tangent where its start and end point meet. Open b-spline curves are always non-periodic.
Because non-periodic elements have vertices or endpoints, they have a physical start point that can be selected when constructing lofted or swept surfaces.