- Polyline: This choice applies to "modern," so-called "light-weight" LWPOLYLINEs. If you have PLINETYPE set to 1 or the default value of 2, then the PLINE command will make this type. (If it is set to 2, and you open an R14-format drawing (or older), any existing polylines will be converted to the the "new" format; if it is set to 1, existing polylines from R14 or older format drawings are not converted.)
- Polyline (2D): This choice applies to the old format polylines. You have to set PLINETYPE to 0 to create new polylines in that format. Unless you have a compelling reason to do so, I would not recommend that. The LWPOLYLINE format results in smaller file sizes and faster processing.
- Polyline (3D): This choice applies to 3D polylines created with the 3DPOLY command. Polylines created by the PLINE command are "flat" or 2D; all vertices have the same Z-coordinate in the UCS that was current at the time of creation, set by the first point selected. In a 3D polyline, the Z-coordinate of each vertex is independent of those of the other vertices.
If you select a polyline to which the Polyline (2D) choice applies, the Properties palette will show it as "2D Polyline" at the top. If you use the LIST command, it will indicate that it is a "POLYLINE".
If you select a polyline to which the Polyline (3D) choice applies, the Properties palette will show it as "3D Polyline" at the top. If you use the LIST command, it will indicate that it is a "POLYLINE".
Here are the Automatic Properties that are available with each type. Note that the Polyline and Polyline (2D) have the same Automatic Properties; Polyline (3D) has some of the same, but lacks the Closed, Elevation and Thickness properties.