Currently just support (as far as I know) for offline syncing of your local files (as in, local to your Roon server).Like Tidal? Or
I've got a home server running Roon, and a network share with music files that Roon streams. In Roon ARC on my phone, I can offline those files and listen even if I'm away from home with my phone, and don't have network access. That's offline syncing from one source (your self-hosted files).