The slice of the Bitcoin chain stored on this device. Each block was fetched once, verified by the engine, and kept here so it never has to be fetched again. As bitcoin-kernel grows toward a shared mesh, this is what your node can serve to others.
Peering (experimental). Turn on "Peer with others" and your browser joins a swarm on a tracker you provide and serves the blocks it holds to other peers directly — the chain held by the swarm, not one explorer. There is no default tracker; bring your own (a JSS /.webrtc endpoint), and the ?tracker= link is the invitation — share it and whoever opens it joins your swarm. Every block is still verified by hash on arrival, so a peer can never hand you a bad one. Storage is shared with the browser and bounded; old blocks drop as new ones arrive.