Is there any? I for one can't tell. I almost always start by zoning medium density, however, upgrading the zone to highest density doesn't seem to do anything.
I just can't recall ever seeing any industrial buildings that show up in high-density but not medium. (I thought smelters were for a while - but no - they do show up in medium)
Why is it there? What difference does it make? How can I purposefully grow "dense" industry if it does in fact exist?