The file systems of iOS and Mac both use HFS+ as file system, with only one small difference. iOS uses case-sensitive file names, Mac doesn’t by default. Both have a feature called “Extended File Attributes” that allow you to set custom values by key.
Apple generally ignored this functionality and it was only briefly – in 5.0.1 – that they actually used an extended attributed for something. The “com.apple.MobileBackup” extended attribute served as a stop-gap-measure to mark files that should not be backed up. Though this was very short lived.
I was facing the problem myself that I needed to save the ETag for an image that I downloaded from the web somewhere. And I wanted to do that elegantly, somehow together with the file itself and in a way that would simply work.