Question: what is the difference between a 2d polyline and a polyline?
Answer: polyline – this is an LWPOLYLINE entity, always 2D. 2d polyline – this is a POLYLINE entity, also 2D. 3d polyline – this is a POLYLINE entity, usually 3D, with each vertex at a different elevation. It is possible to have a “3d polyline” and have each vertex at the same elevation.