Glossary
System Data
The Storage category macOS uses for files that do not fall into the named buckets.
In System Settings, the Storage breakdown lists Applications, documents, and other named groups. System Data is what remains: caches, logs, local snapshots, and other files the overview does not itemize. It is not one directory you can open and understand at a glance.
That is why a single number labeled System Data is a poor deletion target. Disk Housekeeper always previews the actual items in a clean. If you are trying to free up space on a MacBook, start from the preview and from free space in the menu bar, not from the size of this category alone.