If you have a face of a solid surrounded by walls, you can replace the face with a rib fillet.
The new fillet is tangent to the walls and its base touches where the face used to lie.
To create a rib fillet:
Individual faces — Only selects the face clicked.
Adjacent continuous faces — Selects the face clicked and all faces that are adjacent and tangent-plane continuous to the clicked face.
All continuous faces — Selects the face clicked and all faces which form a tangent-plane continuous surface with the clicked face.
A Rib fillet feature icon representing the operation appears in the solid feature tree.