Short answer: creating and holding your identity works offline; most of what you do with it needs a connection.
What lives entirely on your device: your Core ID and your aliases are biometric-secured credentials stored on your own hardware, not fetched from a server every time you need them. The cryptographic material itself doesn't disappear the moment your connection does.
What actually needs a network: signing in to an application, checking a badge, reading or writing key-value data to your Core ID or an alias, and syncing with UIID Cloud all involve a round trip. If you're offline, these simply wait until you're back online — nothing is lost, but nothing completes either.
Why this matters beyond personal convenience: VIVON exists specifically because Web4's ownership model doesn't mean much if the connection underneath it is unreliable or controlled by someone else — but VIVON is still in closed beta, and most people today reach UIID over a conventional mobile or home connection, with the same gaps that implies in low-coverage areas.
If you're setting up a Web4 @home deployment for a village, community or grassroots group with limited or intermittent connectivity, that is precisely the scenario Web4 @home on a Raspberry Pi was built for — see pi.linkspreed.com — but note that your members' UIID sign-in still depends on their connection reaching the network, not just the local server being reachable.
A practical note for travel or remote work: biometric sign-in itself (unlocking the credential on your device) doesn't require connectivity. It's the handshake with the application or the UIID service that does. If you know you'll be offline, sign in to anything you'll need before you lose the connection.
No offline-first mode exists yet for applications built on UIID key-value storage — if your product needs to function fully offline and sync later, write to [email protected] with your use case; this is exactly the kind of demand that shapes what gets prioritized next.