How it decides
The four safety levels
The rating is what cleaning a category costs you, not how much it frees. It is not advice printed beside a button — it decides what the button is allowed to do.
Every one of the 44 categories carries one of these four, and the app behaves differently for each. A Move to Backup category has no bulk clean whatever you click; a Protected one has no delete at all. That is the point of rating them: a level that only changed the colour of a label would be decoration.
Safe to Remove
Regenerates on its own. Nothing of value is lost.
Caches, logs, downloaded packages and build trees — things your Mac or your tools will simply make again the next time they need them. Cleaning one costs you nothing except the seconds it takes to rebuild.
This is the only level the scheduled clean may touch. Clean All Safe takes every category at this level in one go, and the automatic clean takes the ones you leave switched on. Even here the Trash is never emptied automatically: it is the undo for everything else.
Application Caches · Xcode Derived Data · NPM Cache · Google Chrome Cache
Review
Costs a rebuild, a re-download or an app restart. Worth a look before you clean it.
Nothing here is your data, and nothing here is gone for good — but it is not free either. Removing a simulator runtime means downloading several gigabytes again when you next need it; clearing a cache an app is holding open means restarting the app.
Clean takes the whole category after a confirmation that says what it will cost. The difference from the level above is the confirmation, and the expectation that you read it.
Simulator Runtimes · Duplicate Files · Duplicate Pictures · Project Build Folders
Move to Backup
Your data. Move it to another drive, or pick items one at a time.
Files you made or were sent, that nothing will recreate. There is no bulk clean at this level at all — the Clean button opens the list and hands the choice back to you, because the app has no way of knowing which of your own files you still want.
Deleting is rarely the right answer here, which is why the level is named after the other one: Move to… frees exactly the same space and loses nothing.
Large Files · Xcode Archives · Codex Sessions & Plugins · old project checkouts
Protected
Cannot be deleted here. It shows you what to do instead.
Measured, sized and reported — and the app will not remove it, however carefully you ask. These are the places where it cannot tell a cache from the only copy of somebody's work, and a category that cannot tell should not offer the button.
Your Photos library is one: deleting it in Finder can lose photographs, so the row points you at Photos' own Optimise Mac Storage instead. Application Support is another — for many apps that folder is the data rather than a copy of it.
Photos Library · Application Support · macOS Temp Data · Chrome passwords and sign-ins