There’s hardlink, and then below that there’s the COW/dedupe version called “reflink”. Two files can point to the same chunks of data (extents), and altering one does not alter the other. Two files can point to just some of the same chunks of data, too. I don’t think there is much indicator for when this is happening, besides the free space vs used space accounting looking crazy. If you “compsize” two reflinked files at once, it’ll show you the difference.