Some cloud storage service providers offer services that allow you to sync a local folder to "the cloud", and optionally choose to store those files only online, thus freeing up space on your hard drive. Some services that currently offer this functionality include:
- Dropbox Professional's "Smart Sync" feature
- Microsoft OneDrive's "Free up space" feature
- iCloud Drive's "Optimize Mac Storage" feature
- Google's "Drive File Stream" feature
Files that are only available online will typically have a "cloud" icon or badge in the Finder, e.g. iCloud: 
and Dropbox: 
When you choose to have these services store your files only online, do so with the understanding that it's not possible to maintain a local backup of those files.
Online-only files can't be backed up
When you specify that a file stored by one of these storage services should reside only online, the local copy of your file is deleted from your Mac and replaced with a 0-byte placeholder file. If you attempt to open the placeholder file, the agent software for your storage service provider automatically downloads the data of the file to your Mac and the document opens. While this is a convenient feature that allows you to free up some space on your Mac, this feature removes files from your local storage, which means that CCC can't make a backup of these online-only files. Before using these online-only features, you should consider whether you are comfortable with not having a local backup of the files that you choose to store only in the cloud.
Placeholder files may be backed up, but may not function as placeholder files on the backup disk
As noted above, when you open a placeholder file in the Finder, the agent software downloads the original. Likewise, if you attempt to copy a placeholder file via the Finder from one volume to another, the agent software downloads the data to the source, then copies the original file (leaving the downloaded source file in place). CCC backups do not behave like Finder copies. And for good reason – if you have 1TB of online-only files on your 500GB SSD, you wouldn't want Dropbox or iCloud to download all of that data when CCC attempts to make a backup! Rather, CCC copies the placeholder files as they are, retaining all of the placeholder attributes of the source files. CCC makes a non-proprietary backup of your files; our goal is to make the destination files look exactly like the source files.
Some placeholder files won't function as placeholder files on the destination. OneDrive, for example, won't see a placeholder file that is outside of your OneDrive folder (i.e. on your startup disk) as a "true" placeholder file, and will not engage to download the file's data when you attempt to open it. CCC makes a best-effort attempt to not copy OneDrive placeholder files at all. Microsoft's OneDrive client software actively prevents applications from accessing those files — OneDrive placeholders are impossible to back up.
Dropbox's placeholder files function correctly when you back them up from an APFS volume to another APFS volume, and when you back them up from an HFS+ volume to another HFS+ volume. Because Dropbox uses a different proprietary technique for creating the placeholder file on each volume format, though, these placeholder files won't behave correctly when transferred from one filesystem format to another.
Google Drive File Stream uses yet another proprietary device for its placeholder files. These placeholder files (which include all Google document formats) can't be opened by any application other than Google Drive, so CCC does not attempt to back them up.
OneDrive may delete online-only files from the cloud when you restore a OneDrive folder from a backup
Because Microsoft's OneDrive syncing software prevents applications from accessing the contents of OneDrive online-only placeholder files, those placeholder files cannot be present on a backup. If you restore a OneDrive folder from a backup, the OneDrive service should be smart enough to not delete files from the cloud simply because the placeholders are now absent. For comparison, Dropbox and iCloud won't delete files whose placeholders are absent, rather those services will only delete a file from the cloud when an actual file removal event occurs. In our own testing, OneDrive does not delete online-only files from the cloud when restoring from a backup. People have reported this concern on Microsoft's forums, and two CCC users have reported the same concern to us. If you restore from a backup and encounter this problem, you can restore the deleted files using these instructions from Microsoft.
If you're reading this because you encountered an error in your backup task indicating that CCC dropped a OneDrive placeholder file, take a moment to consider whether you should continue to include your OneDrive folder in your backups. On one hand, having a local backup of the OneDrive files that actually reside on your Mac is really nice to have. On the other hand, if you don't recall that placeholder files will be missing from your backup, then you may have some hassle in the future if/when you restore from that backup.