When space is low, review Storage recommendations and individual files instead of deleting a folder because its name sounds temporary. “System Data” is a category, not one folder with a universal cleanup rule.

Check each candidate

Open Apple menu > System Settings > General > Storage. Open a large file or old download and check what it is, whether you still need it, and whether another copy can be restored. Verify a cloud or external copy by opening it. Apple’s Store in iCloud, Optimize Storage, and automatic Bin emptying have different effects; read the description before enabling each option.

Example: an old installer

Suppose Downloads contains a 2 GB installer for an app already installed. Confirm the app opens, decide whether you may need the installer again, and check for another copy. It is a deletion candidate only if it is replaceable and no longer needed. A unique work export is not disposable just because it is large. Storage totals can take time to update.

If System Data stays large

macOS can clear some safe caches and temporary files when space is needed. Macworld lists caches, logs, temporary files, and app support data among possible causes, which vary by Mac. Do not delete broad Library folders without identifying the contents and recovery path. If a disk problem seems likely, first read what Disk Utility First Aid checks. See Apple’s Storage options and Macworld’s current System Data guide.