We simply need 90d C terminals and be rid of all the confusion.
JAP>
90C terminals do exist, but equipment built to allow you to take credit for this rating is uncommon. And it's not just the terminals that need to be 90C rated, but the equipment as a whole. You may very well see lugs on your equipment marked AL9CU, which would appear to indicate this, but that isn't enough. Not only that, you'd have to have 90C rated equipment on the other side as well.
One example where you do get to use 90C for terminal ratings, is if you have separately-installed connectors, in an enclosure that otherwise empty. Such as split bolts, Polaris blocks, insulation piercing connectors. It could be a way to recover from an error, or a value-engineering decision, to locally use 75C sizing at your equipment, splice to 90C sizing in an adjacent enclosure, and use 90C sizing for the majority of the length.
Another example is Mike Holt's illustrations show this used when separate lugs are bolted to busbars. I would like to know what kind of equipment allows this rating? Is it just equipment that is nothing but busbar, that would allow this? Switchboard sections that are only busbar within the section? Many transformers have busbars with separately-installed connectors, but does the fact that it is in a transformer restrict you to 75C?