Converting 2D elements for use in 3D.
Any 3D element(s) can be saved as an object with the Save Project As... menu command. (In Archicad 11, Save 3D Model As...) This technique is known as 'slabifying' since such models are often built from slabs. Objects saved in this way are dumb (not parametric), but it's still a useful trick.
2D elements can't be saved this way, because they never appear in the 3D window, where 3D object saving takes place. Despite the fact that GDL contains commands for 'flat' shapes in 3D, including LIN_ (a line) and PLANE (not a joinery implement). But there is a workaround for 'slabifying lines'. When you open a 2D DWG as an object, 2D lines are created as LIN_ statements in the 3D script. When you place the object in the model, you get the 2D geometry in 3D.
It's that simple at it's simplest, but real world applications need some tweaking. In this example, I'm converting an Archicad library 2D tree elevation symbol so I can use it in a sketch render image. Other applications might be a complex ornament in a hidden line elevation, or a busy glazing design placed in front of a conventional window.