A proprietary format; okay, that makes sense then. I can't expect ImgBurn to work correctly with something like that.
Dang; that means I'd have to track down some old disc somewhere that probably doesn't work on my modern computer, or download a trial version that probably won't include the feature I need, or outright buy software I'd never use again. ...Actually I could just get on eBay and buy another copy of the disc I want to burn for less than what it would cost to buy Alcolhol 120%. These people need to rethink their business model.
Thank you for the help/information.
It's a relatively minor thing, but I would suggest editing the error code to say "Only single track images are supported with this format" or possibly a more informative "This is a proprietary format and we can only support its single-track images. You will need to use the original software that created this file." The current method of selecting a file that is listed among the supported file types (in windows' open dialogue) and then being told that the format isn't supported is a little confusing. I get what it was saying now, but at the time it looked more like an error.
Again, thank you for clearing that up.