Apple has long offered two versions of the same HFS+ partition formatting scheme used to create a filesystem for a Mac-mountable volume: “Mac OS Extended (Journaled)” and “Mac OS Extended (Case-sensitive, Journaled)”. With the addition of APFS formatting, that flavor is also available in case sensitive and insensitive versions. What a difference that “case-sensitive” makes!
Case-sensitive filesystems can allow multiple files to have the same human-readable name using different capitalization. Blue dolphin.pdf and blue dolphin.pdf and bLuE dOlPhIn.PDF are all considered different items to a case-sensitive filesystem. With the opposite, a case-insensitive filesystem, the default option for macOS in HFS+ and APFS, those files can’t co-exist: They’re all effectively the same name with a different appearance.
