That's a left-over from the very first Amiga, the A1000. The icon type denotes the kickstart disk. If you want to say so, that is a "ROM image", but it is actually a kickstart disk. It is no longer in use, apparently.
Thanks Thomas. I suspected it could be the Kickstart disk. I'm suprised that a Kickstart icon type even exists though. I thought the Kickstart disk would just be a normal disk type, perhaps one with no visible files on but that said "100% Full" so that no files could be accidentally copied onto it.
So when you select "Info" from the Icon menu on Workbench on a Kickstart disk, what does it greet you with?
P.S. So presumably Workbench won't let you format a Kickstart disk...it makes sense now why they created that particular "Kickstart ROM" disk type.