^See above for said smarter guy than meRetrofocus wide-angle lenses of the shortest focal lengths tend to produce a curved field of focus about the subject at extreme close focus. It’s shaped like a portion of a sphere where we expect a lens to produce a flat plane of focus. Some of the better lenses of this type use a single element or two-piece compound element between the front and rear optical units that moves independently of the rest of the lens. The independently moving unit is known as a field flattener. By moving it to the proper position inside the lens assembly, the curved field of focus is flattened as we’d expect of a good lens. The independently-moving field flattener is sometimes described as a “floating element”.
These are moved as necessary by a cam assembly as you turn the focusing ring on helicoid-focusing lenses like 35mm SLR lenses. But the RZ67 is a bellows-focusing camera. In this case we have to turn a “float ring” to manually adjust the position of the field flattener after first focusing the lens. Some close-focusing macro lenses also have an independently-moving field flattener unit. Some examples are the 105mm Micro Nikkor and the Mamiya 140mm macro lens for the RZ67.