iCloud > Documents and Desktop
Well, that's disconcerting - one of the 'features' of the new OS is for the iCloud service to be able to auto-sync your Documents & Desktop files to it . If you didn't deal with it during the installation process (love the idea of offering a new service during install, without any explanation as to what it means. But I digress....), you can access it via System Preferences > iCloud > iCloud Drive > Options. I left it till after the install and explored the option from there.
Not unreasonably (I thought) I expected that it simply uploaded copies of my Documents and desktop to the service, leaving the originals in situ on my computer. Not so, as I soon discovered. I went looking for my Documents folder in my User folder on another matter and found it gone! Everything was moved to the iCloud folder (as now found in the Finder sidebar) and opening / saving files from the desktop shortcuts was accessing them from there - and from there, then sync'ing to the iCloud service.
I want my computer to have my files on it. In a folder created for that purpose. In my User Folder. Not elsewhere. Sure, I 'get' there is a folder with my stuff on it elsewhere still on the computer and that's what syncs with iCloud. I like the idea of backups - always have. I use Dropbox, but again, I manually copy the files I want backed up to it's folder - I don't expect (or want) to use it as my primary storage location. And that's not what I wanted from iCloud.
Reverted back with some hassle - disabling this 'feature' returns a Documents folder to your Folder - but empty. Some copy / paste involved to get back to where I wanted to be.
Don't expect me to be happy with telling me what I really want, Apple.