From FreeCAD Documentation
- Fill Extrusion: When the Thickness value is not null, this will generate lofting faces (the blue faces in the above screenshot).
- Fill Faces: This will fill all the geometrical figures closed in faces (the red faces in the above screenshot).
- Offset: This will push the shapes mapped above into the target face. Do not put an offset greater than the thickness, it makes the face disappear on the inside.
- Thickness: If not null, this will give thickness to the surfaces created above.