In a specific version of Macintosh PC Exchange, or by installing the 720K HFS Formatter extension, a Mac user could format a double sided disk as 720K with HFS file system rather than the usual 800K.

However I'm completely at a loss what this capability was used for. Apple hardly even acknowledged it, and it's not mentioned in the manuals at all.

Is it for an Atari ST with the Mac with the Spectre GCR? Is it for an Amiga? I could ask in their respective forum communities, but I figured a general query would be prudent first. Is it for a PC Compatible? There weren't many software things that I know of that could read HFS, unless I'm seriously mistaken. This would be like the 486 to Pentium I era.

This PC Exchange version came only out with the Quadra 610 DOS Compatible, and not any other version. The Formatter extension, if you didn't use PC Exchange 2.0DC, only came out with the System Update 2.0.1, and isn't part of System Update 3.0.