How it decides
Every item gets one of three verdicts. Anything the app doesn't recognize is treated as Keep.
- Things that rebuild themselvesBrowser and tool caches, logs, build files, and what is already in the Trash.Safe to delete
- Things you might wantDownloads, device backups, apps you don't use, and data left by deleted apps. They go to the Trash, so you can get them back.Check first
- Your files, and what macOS needsDocuments, photos, messages, app data and system files.Keep
What removal never does: remove anything you didn't select, remove files outside your home folder and Applications, follow a link out of your home folder, or remove files of an app it can see is open. Items marked Keep can't be selected (except an app's own data when you uninstall that app).
Questions people ask
What will StorageJanitor delete?
Only items you select. Items marked Safe to delete are deleted: apps rebuild them, or they were already in the Trash. Items marked Check first are moved to the Trash. Items marked Keep can't be selected (except an app's own data when you uninstall that app).
What happens to a folder it doesn't recognize?
It is shown with its size and treated as Keep. Nothing is offered for removal unless the app knows what it is.
Does it delete system files?
Not as files. Removal only works inside your home folder and the Applications folder, and never on files owned by macOS. The full version has two separate fixes that ask macOS to do the work: removing local Time Machine snapshots and rebuilding the Spotlight index. Each says what will happen before you confirm.