Cone is defined by 2 open curves (Arcs, Polylines and Splines). Curves can be on different UCS or even 3D curve (3Dpoly, 3Dspline). Each curve is divided into "Number of Segments" lines that are connected by 3D faces.
Elbow is curved pipe. Centre of pipe cross-sections (= arcs) lay on circle. It is represented by a number of cones connected at cross sections. Elbow is defined by Spiral radius, Inlet radius, Final radius, Spiral Gap (for welding), Angle and Number of cones.
Spiral is curved pipe. End points of cross-sections (= arcs) lay on circle. Defined is with the same parameters as Elbow, just Gap is usually bigger. This geometry is for turbine inlet spirals or radial fan housings.