It is the catch-all category in Storage settings for everything macOS couldn't file under a more specific label. Here's what's inside it, and how to shrink it without breaking anything.
The Storage pane (System Settings → General → Storage) files everything on your disk into categories. Most are self-explanatory. Two are not: macOS, which holds system applications and files, and System Data. Apple describes System Data as containing “files that don't fall into the categories listed here” — primarily log files, caches, VM (virtual memory) files and other runtime system resources, plus temporary files, fonts, app support files and plug-ins.
Two details from that documentation matter in practice:
In practice System Data is mostly application support and cache data living in your user Library folder — ~/Library — plus logs, saved state, and memory-swap files. Common large contributors:
~/Library/Caches, per-app container caches.~/Library/Logs, crash reports.~/Library./private/var/vm, sized by memory pressure and uptime, not by your files.Finder and disk utilities report free space in ways that include purgeable space: space macOS considers reclaimable but hasn't actually released. Time Machine local snapshots are the classic example. They are created automatically while your Mac is away from your backup disk, and they hold point-in-time copies of files — including files you deleted. macOS thins them under pressure, but until it does, that space isn't really yours.
You can see them with tmutil listlocalsnapshots / in Terminal, and thin them with sudo tmutil thinlocalsnapshots / <bytes> 4. Treat this as an advanced step and understand the trade-off first: thinning snapshots removes local recovery points.
Because Storage settings won't break System Data down, use a scanner that walks the filesystem and sorts by size. Two things to insist on:
Massive Disk's Space Savers view is exactly this: a curated registry of caches, backups and forgotten files — Xcode DerivedData, iOS DeviceSupport, Simulator caches and devices, Archives, npm/pnpm/bun/yarn/brew/pip/Gradle/Maven/CocoaPods/SwiftPM/Cargo/Go/HuggingFace caches, Docker.raw, iOS backups, Mail downloads, user caches, logs and the Trash — each tagged Safe, Review or Caution so you can tell regenerable build output from something you actually need.
/System, /private/var/db, or macOS's own caches. On modern macOS the system volume is sealed and signed; fighting it is how you end up reinstalling.~/Library you don't recognise. An unfamiliar folder there is often an app's database, not junk. Use the owning app to clear its own cache.Scan once, sort by size, and work down the list — but stage before you delete. Any tool you use should move items to the Trash rather than hard-deleting them, so a wrong call is a drag-and-drop recovery instead of a lost file. Massive Disk has no shredder and no permanent-delete option for exactly that reason: every removal, from the cleanup queue, Space Savers, Duplicates or the uninstaller, goes through the Finder's own Trash.
No — Apple's Storage pane does not let you manage that category, and most of its contents are actively used by macOS and your apps. You shrink it by clearing specific caches and app data that you can identify, not by deleting the category.
Caches, logs, virtual-memory files and cloud-sync staging data all grow with use. Apple notes the category varies in size depending on the current state of the Mac.
Not by hand. Those folders hold app databases and preferences as well as caches. Clear a cache from within the app that owns it, or use a tool with a curated, safety-tagged list.
Massive Disk is a native Apple Silicon app that maps every byte of your storage across six visualizations — then gives you safe, reversible ways to get space back. Scanning is free forever; cleanup tools are Pro.