iCloud settings on Mac book shows that Dropbox is storing its file inside iCloud when that isn't actually happening I was running through my settings when i stumbled upon this. I'm curious what is actually stored inside iCloud (by Dropbox), because the files inside Dropbox are obviously not (my Dropbox folder is much bigger than iCloud capacity)

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Definitely it's not the data you put on dropbox cloud. I think this old answer hints about your question. I know what time machine back up is but I think this service might later be included in iCloud backups too.

As for iOS, ios backup
details explain what it stores.