In plain terms: cache is disposable copies an app can rebuild, documents and data are your actual stuff, and app size is the program itself. The cache is safe to clear; your documents are not. Knowing the difference is the whole trick to cleaning storage without losing anything.
TL;DR
- Cache: temporary, disposable copies (images, thumbnails); safe to clear, rebuilds itself.
- Data/Documents: the things you created or saved (downloads, messages, login state); clearing loses them.
- App size: the app program itself; removed only by deleting or offloading the app.
- iPhone Storage lumps cache and documents together as "Documents & Data," which hides the difference.
- Clear cache freely; treat documents as keep-by-default.
What is a cache, in plain English?
A cache is a stack of copies an app keeps so it doesn't have to fetch the same thing twice: photo thumbnails, web pages, map tiles, video previews. It's disposable by definition. Delete it and the app simply re-creates it next time you use it. Nothing you care about is lost, you just pay a small re-download cost. Because of that, caches are the safe thing to clear. More on doing this cleanly in how to clear app cache on iPhone without deleting apps.
What counts as data and documents?
Data and documents are your stuff and the app's saved state. Examples: a podcast you downloaded for a flight, photos saved inside an app, your message history, drafts, offline maps you chose to keep, and login sessions. Clearing this is not free, you lose the actual content or get signed out and have to set things up again.
The practical rule: if losing it would annoy you or require re-doing work, it's documents, not cache. Keep it by default.
What does "app size" mean?
App size is the program itself, the code and built-in assets you downloaded from the App Store. You don't clear app size by clearing cache; the only way to reclaim it is to delete the app or offload it. Offloading removes the program but keeps your documents and data, so reinstalling restores everything where you left it.
Where do I see this on iPhone, and what's the catch?
Go to Settings > General > iPhone Storage, tap any app, and you'll see two figures: the app size and "Documents & Data."
The catch: iOS rolls cache and your real documents together into that single "Documents & Data" number. So a large figure could be all disposable cache, all precious downloads, or a mix, and the screen won't tell you which. That ambiguity is why people fear clearing it. See also iPhone storage full but nothing to delete: what's actually using it.
What iOS does natively, and where it stops
iOS automatically reclaims purgeable caches when storage runs low, and offers Offload Unused Apps and Delete App from each app's storage page. Safari has its own control in Settings > Apps > Safari > Clear History and Website Data.
Where it stops: iOS gives no universal, per-app button to clear only the cache while keeping your documents. For most apps your choices are the in-app setting (if it has one) or offload/reinstall. The system won't separate disposable from precious inside that combined number for you.
What this cannot do
Reading these labels won't, by itself, split a 3 GB "Documents & Data" entry into cache versus keepers. iOS doesn't expose that breakdown, so manual cleanup means guessing or using the offload trick. If you want clarity, Cleanor for iPhone helps you spot large reclaimable files and duplicates across your phone, so you target disposable storage and leave your real documents alone.
FAQ
Is it safe to clear an app's cache?
Yes. Cache is disposable by design, the app rebuilds it the next time you use it. The only cost is a brief re-download. Your documents and saved content are stored separately and are not affected by clearing cache.
Why is "Documents & Data" so large for some apps?
Because iOS combines disposable cache with your real saved content under one label. A streaming or photo app may hold gigabytes of cached previews plus your actual downloads, and the screen won't separate the two.
Does deleting an app remove its documents and data?
Yes, deleting removes the app and its documents and data. If you want to reclaim the app size but keep your stuff, offload it instead, that keeps your documents so reinstalling restores everything.
To target the right storage without guesswork, try Cleanor for iPhone or our guide to free up iPhone space.