What does the checkbox for some apps in iCloud > Options > Documents mean? In System Preferences > iCloud > Options > Documents, there are checkboxes for "Apps that store documents and data in iCloud."

For many apps, it's clear what the checkbox means: the app can, optionally, store a document in iCloud that can be accessed from any of my devices.

But there are also checkboxes for Mail, System Preferences itself, and Weather. None of those create documents. In Mail's case, the mail is stored in IMAP accounts, so what would having the checkbox checked mean? What would having the checkbox unchecked for either System Preferences or Weather mean?



In the top-level iCloud system preference, there is a separate checkbox for Mail. That checkbox, when checked, shares the configuration information for all e-mail accounts one has.
Firstly, the checkbox for Mail just means it syncs Mail.app settings via iCloud and also it enables the iCloud Account in Mail.app or turn it off. 

Secondly, System Preferences settings are also synced via iCloud, for example iCloud syncs my preferred Wifi to any new iOS or Mac I enable iCloud on.

Likewise Weather locations for any city you setup is synced over iCloud.