Unit Conversion
The Unit Conversion node can be used to easily change the results when texturing a parameter that uses a physical unit. For example, the OpenPBR material uses centimeters as a fixed physical unit for several parameters like the Transmission Depth and Subsurface Radius. So if you've got a Cloudy with a Chance of Meatballs situation on your hands, with giant food raining from the sky, this node can be plugged into one of those ports and you could use meters for those massive meatballs instead. The Unit Conversion tool makes it easy to work in whatever unit you prefer, maybe you'd rather work in feet or inches?
The Unit Conversion node should only be used when the Units Mode is set to "Project Settings" in the Render Settings > System tab — this uses C4D's Project Scale unit as the Scene Unit. When used in scenes using the old "Custom" mode it will not function as expected.
Examples
Fixed vs Scene Units
Distance based parameters in Redshift come in two flavors, Fixed units or Scene Units. Fixed unit parameters are used for physically based shading phenomena like subsurface scattering and volume transparency depth. These parameters remain constant even if you change your scene scale at the Render Settings level. Meaning if you start working with your Scene Scale in centimeters and switch to meters the previously see through bottle of soda now looks opaque. This is because the bottle model is now 100 times larger but the soda's transparency depth is still only defined as a few centimeters.
This is where Scene Unit parameters come in, these types of parameters automatically scale with the Scene Scale because they are generally used to define more artistic effects like ambient occlusion, a distance shader, or round corners. This ensures that artistically driven utility shaders don't break if you need to change your Scene Scale.
Ultimately the choice is yours because lets go back to that bottle of soda, what if you wanted it to always look the same, no matter the Scene Scale? Then you could use a Unit Conversion node connected to the Transmission Depth set to the "Scene Units" mode and then it would look the same with the Scene Scale at centimeters or meters. Or maybe you want your round corners to behave more like a physically based effect, you could use a Unit Conversion connected to the Round Corner's Radius parameter set to "Fixed" mode and it would no longer scale with the scene.