Mac storage basics · System Data and the storage categories
What is System Data on a Mac?
Short answer: System Data is macOS's catch-all label for caches, logs, local snapshots, virtual memory, app support files and anything it can't classify. Older versions called it Other.
System Data is the catch-all category in macOS Storage settings for everything that doesn't fit Applications, Documents, Photos, Mail and the other named groups. It typically includes caches, logs, local Time Machine snapshots, virtual memory and sleep files, app support data, and some files macOS can't classify.
It isn't one folder you can open and empty; it's a label on many places at once. Some of it is genuinely needed by macOS, some is rebuildable (caches), and some is temporarily held (snapshots) and releases on its own.
Older macOS versions called this Other. Storage Studio's What's Using Space and Local Snapshots tools name the biggest parts of it individually. See why System Data gets so big.