Kinematic elements are coplanar with the XY-Plane of the active Mechanism-Editor*.
Kinematic elements represent the moving parts in your machine. You join Parts to each other to form kinematic-chains (mechanisms, linkages).
Kinematic elements are NOT solid bodies; and they do NOT have shape or form**.
* An exception is a Part with a Ball-Joint at its start-Point and end-Point.
** To add a solid body to a Part, add to it a Profile and/or import a SolidWorks part/assembly document or an STL file from other CAD.
The Kinematic elements toolbar is to the left of the graphic-area.