Solid Edge Rib fails when profile sketch is defined on a face

In this example, the user has defined a Rib by offsetting the parallel sketch plane from the floor of their part by 3mm.

The Thickness of the Rib is 3mm and Depth (direction upwards) is 5mm.

When the user defines a distance of zero for the offset of the parallel plane, the rib will fail.

Why is this?

Ribs are defined by sketches in space - hence why the Parallel Plane is the default Sketch plane.

A Rib is defined by taking the sketch profile, then extending the geometry of the feature to the adjacent surfaces.

If you define the profile as being on a face - the floor of the part, in this instance - consequently, the Rib has no contextual adjacent faces that it can extend and trim to.