Your crypto-fiat fintech custodies user crypto. Your INTERNAL ledger records every user's BTC balance (off-chain, just SQL rows). The ACTUAL chain holds the UTXOs / account balances. These two views MUST agree to the satoshi. Discrepancies happen: a chain reorg invalidates a deposit you already credited, a dust transaction adds satoshis you didn't book, network fees deducted at broadcast time aren't reflected in user wallets. A reconciliation job runs every block (every ~10 minutes for BTC) comparing the two views and posting CORRECTION entries. This lesson walks one such correction: chain shows 0.002 BTC less than ledger; you book a 'reconciliation adjustment' to bring them back in sync.