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Windows 10 use a column in File Explorer (file property) for icons instead of overlay
Dropbox uses icon file overlay to show file sync status in File Explorer in Windows 10. Windows has a limit of just 15 overlay icons and different applications push other appliations out of the top ...
What Rich said is no longer true. iCloud for Windows also makes use of this feature, as it syncs iCloud Drive and iCloud Photos to the computer using smartsync-like functions, and the items use the same Status column (and even the icons) than OneDrive.
With this, I assume that the API is no longer privative.