Create 2 rectangles using the line rectangle tool
click polyline->make polyline from segments
click on both rectangles
click polyline->append node
click one rectangle
This results in one closed polyline rectangle and one open polyline rectangle, I think.
I use Qcad to design panels with holes in them that I often import into Blender to make 3D product mockups. Importing this DXF file into blender gives me one rectangle that's a closed surface and one that's not. The problem is that there's an extra segment with length 0 in the closed one, which sometimes messes up blenders calculation of the surface. Why doesn't "make polyline from segments" result in a closed polyline automatically?
It's possible that it does, and this is a problem with the blender import script, but I'm not sure. If I could import closed curves into blender from Qcad it would speed up my workflow immensely. I spend a lot of time closing all my curves after an import.