The System volume (the one without Data appended to it), introduced in macOS Catalina, is a read-only volume that stores almost all of macOS Catalina and its preinstalled apps. That’s everything (minus most of the OS) stored on your Mac, which includes your user account, apps, documents and other files. TL DR the "About This Mac" gives the "real-world" breakdown, and you only need to worry about the Disk Utility stats if you are manually partitioning drives to meet certain targets, or doing other low-level work. This is actually a bit of a nightmare in certain cases, for example when it comes to writing custom utilities to display the disk usage, because I haven't yet managed to find a way to replicate the data shown by "About This Mac" (I end up only being able to show the Disk Utility version). Because the "About This Mac" is collecting the information with this in mind, it displays a more context-aware story with regards to the disk usage.
This is similar to how modern applications expand their RAM usage to fill what is available and cut it down when necessary (next time you're going to start working, open Activity Monitor and take a look at the Memory usage of the Window Server process it'll go from >4GB to however small it needs to be, depending on how much other stuff is going on). However, the OS manages these files silently for you, and if you need more space it will arrange matters so that space will appear (up to some limit, obviously). These files take up space on your drive, hence why they show up on Disk Utility because it looks at it from a really low-level perspective. I can't tell you right off the bat what "other" includes, but this is a more accurate reflection of the usable space on your drive at the moment.īasically, there are a bunch of files that macos uses "in the background", as it were, related to system and application caches, time machine, etc.