Snapshot baseline recovery after client acknowledges no recent snapshot

Source engine multiplayer clients acknowledge snapshot sequence numbers to keep state consistent. When a client acknowledges a snapshot older than the server's current baseline, the engine must decide whether to resend the full snapshot or apply a re-baselined delta against an older acknowledged baseline. The exact rule for handling baseline recovery when a client acknowledges no recent snapshot — such as after a prolonged disconnect or network partition — is not uniformly documented across Source titles (e.g., GoldSrc vs. Source 2). This uncertainty affects snapshot integrity and client-side prediction reconciliation. How does the engine choose between a full resend and a delta-resend when the client's last acknowledged snapshot is stale or missing? What mechanism governs this baseline recovery decision?