Open curves can be used for solid base extrudes or solid cut extrudes. They cannot be used for solid add extrudes. For base extrude solids solids, a sheet is created without any end caps.
For cut extrude solids, you must ensure that the cut will divide the solid into two distinct pieces like in this figure.
When working with open curves, an additional checkbox called, Flip side to cut, is displayed. After previewing your result, you can check this checkbox if you want to keep the solid on the other side of the extrude.
