What is System Data on Mac?

Open System Settings › General › Storage and there's a big grey bar called System Data. Here's what's actually in it, and what you can safely do about it.

The short answer

System Data isn't a folder. It's a leftover category: macOS counts everything that doesn't fit its other buckets (Apps, Documents, Photos and so on) as System Data. Apple describes it as files used by the system such as log files, caches, VM files and other runtime resources, plus temporary files, fonts, app support files and plug-ins.

That's why it can be huge, and why Storage settings won't let you open it: Apple says you can't manage its contents from there.

What's usually inside

What	Where it lives	Safe to remove?
App caches	~/Library/Caches	Usually yes. Apps rebuild them, though they may be slower the first time. Details here.
App data	~/Library/Application Support, ~/Library/Containers	Careful. This is apps' real data (settings, libraries, chats). Clean it up from inside the app.
Xcode build data	~/Library/Developer/Xcode/DerivedData	Yes. Xcode rebuilds it. Details here.
iOS device support files	~/Library/Developer/Xcode/iOS DeviceSupport	Mostly. Folders for iOS versions you no longer test on can go. Details here.
Simulators	~/Library/Developer/CoreSimulator	Unused ones, yes. Remove them from Xcode or with xcrun simctl delete unavailable. Details here.
iPhone and iPad backups	~/Library/Application Support/MobileSync/Backup	Old ones, yes. Manage them in Finder (select your device › Manage Backups). Details here.
Time Machine local snapshots	Managed by macOS	Leave them. macOS removes them on its own when it needs space.
Logs, VM and swap files	System folders	Leave them. macOS manages these.

Everyone's mix is different. A developer's System Data is often mostly Xcode and simulators. For most people it's app data, caches and old phone backups.

How to clear it safely

1. Find out what yours actually is

Before deleting anything, see which of the above is big on your Mac. Guessing is how people delete things they needed.

2. Clear what rebuilds itself

Caches and build files are the safe wins. Apps and tools recreate them when they need them.

3. Clean up app data from inside the app

Photos, Messages, Mail and chat apps store their data in System Data. Deleting their folders directly can break them, so use their own settings (for example, Messages › Settings › Keep messages).

4. Start up in safe mode once

Apple suggests starting up in safe mode to clear some system caches. macOS rebuilds them as needed.

What not to do

• Don't delete folders in /System or /Library because they're big.
• Don't empty folders you can't identify. If you don't recognise it, leave it.
• Don't expect the number to hit zero. Some System Data is always there and is supposed to be.

Why the number in Storage settings doesn't match other tools

macOS calculates its categories its own way, including shared APFS space, snapshots and purgeable space. Any tool that measures files directly will get a somewhat different total. What matters is finding the big items you can act on.