Indeterminate

An observer is wired and has reported, but its signal cannot distinguish reachable from not.

The browser is the reason this value exists: navigator.onLine === true means only that the user agent believes it has an interface, not that anything is reachable across it — it is famously true on a captive portal, on a LAN with no uplink, and on a machine whose VPN just died. Reporting Reachable from it would be a confident lie, so BrowserConnectivityObserver reports this instead, and toAvailability carries it through as FabricAvailability.Unknown. The negative direction is trustworthy, so navigator.onLine === false is Unreachable.