: If you have a planar opening (a flat hole) in a polysurface, the Cap command will create a flat surface to seal it and turn it into a solid.
To ensure your feature is truly "solid" (water-tight for 3D printing or boolean operations): Rhinoceros 7.24.22297.11002 RC
: When extruding a closed curve (like a circle or square), ensure the Solid=Yes option is selected in the command line to create caps at both ends. Verification Tools : If you have a planar opening (a