What System Data can include
The total can include caches, logs, application support files, local snapshots, developer tools, device backups, virtual machine data, and other files that do not fit a simpler category.
Why the number can change
macOS can recalculate storage categories over time. Temporary data, snapshots, application activity, and incomplete indexing may also affect what appears in the summary.
Find the actual folders
Use DiskPal to scan a permitted folder or volume, then follow the largest map regions. Paths and ranked child folders are more actionable than a single System Data total.
Do not delete blindly
Some system and application files are essential. Inspect the path, identify the app or tool that created the data, and prefer that product’s own cleanup controls when available.