Probabilistic finality
On proof-of-work and some proof-of-stake chains, each additional block makes a reorganisation exponentially less likely but never impossible. Finality is a confidence level you choose by waiting.
This is why a deposit of a large amount requires more confirmations than a small one: the receiver is choosing how much reorganisation risk to accept.
Economic finality
Ethereum finalises checkpoints roughly every two epochs. Reverting a finalised block would require a very large amount of staked value to be destroyed, which makes it economically irrational rather than merely improbable.
Between inclusion and finalisation there is a window of a few minutes where reorganisation is possible, which matters for high-value settlement.
Fast chains and rollups
High-throughput chains confirm quickly and define finality differently; some provide it in under a second, others rely on validator supermajorities. Rollups add another layer: a transaction is confirmed by the sequencer instantly and is only as final as Ethereum once the batch settles.
This gap is why a rollup transaction can feel instant while a withdrawal to Ethereum takes much longer.
What to do with this
For ordinary swaps, sequencer or block confirmation is sufficient — the trade executed and the tokens are in your wallet. For large transfers, for accepting payment, or for anything you cannot repeat, wait for the chain's stronger finality guarantee.
FBT Swap shows the transaction hash so you can follow confirmation on the network explorer rather than guessing from the interface.