Diffusion roughly translates over to Highlight. While there are several work-arounds, Carrara does not currently handle true diffusion. Diffusion can be defined as "fuzziness" of reflected light or how sharp and clear the light bounces off of the edges of an object. This also impacts reflections themselves; whether the reflection is a clear duplication of the image or blurred like it would be on brushed aluminum. The main work around that I use is to apply a grunge map or some kind of irregularity into the highlight channel to vary the brightness of the reflected light. Then that combination is again repeated in the reflection channel.You can also play with variations of that in the Bump and Shininess channels if you like. Hope that this helps a little. Mark