The center curve is open. To create a
rib you can use open profiles and closed center
curves as well. 3D Center curves must not be discontinuous in tangency. You can
also use planar wireframe geometry as your profile or center curve.
It is
recommended that the profile should be on the center curve in a plane normal
to the center curve. Otherwise, it may lead to an unpredictable shape.
The app now previews the rib to be created.