A rib is a member type of RFEM that is used to consider downstand or upstand beams with effective slab widths of the corresponding surfaces. For this purpose, a member is defined in the area of a slab, which initially rests in the plane of the slab. In the detailed settings of the "Rib member" type, further entries are possible and required for the member: In the "Rib" tab, you can specify the arrangement of the rib on the positive or negative side of the surface, and define the width for the integration of surface internal forces.
Using these entries, you can design the rib as a downstand or upstand beam with effective widths in the Concrete Design add-on.