A transaction shown as confirmed has been included in a block. Whether it can still be undone is a separate question, and the answer depends on how the network reaches agreement.
Inclusion is not the same as permanence
Being in a block means a producer accepted the transaction and built it into the chain at that height.
If a competing branch later becomes the one the network follows, that block may be dropped and its transactions returned to the pending pool or invalidated entirely.
So confirmation is evidence rather than a guarantee, and the strength of that evidence grows with each block built on top.
Probabilistic finality never quite reaches certainty
In networks where block production is a race, reversing a block requires outproducing the honest chain from the point of the change.
The cost of doing so grows rapidly with depth, so after several blocks a reversal becomes impractical rather than impossible.
This is why services wait a set number of confirmations before treating a deposit as settled, and why that number varies with the value at stake and the security of the chain.
Economic finality names a price
Proof of stake systems can do something different, by having validators vote on blocks and penalising any validator who votes for conflicting histories.
Once enough stake has committed to a block, reversing it would require a large share of validators to sign contradictory votes and forfeit their deposits.
Finality here is not a matter of accumulated work but of an identifiable cost that an attacker would have to bear, which can be stated as a quantity of stake destroyed.
Layers inherit the finality beneath them
A rollup can confirm transactions to its users almost instantly through its sequencer, which gives a fast and useful signal.
That signal is a promise from the sequencer until the data and proofs are settled on the base chain, at which point the guarantee becomes the base chain's own.
Withdrawal delays and challenge windows exist for exactly this reason, and they are why moving funds out of a layer takes longer than moving them within it.
Why the distinction matters commercially
Anyone releasing goods, credit or another asset against an incoming transfer is choosing how much reversal risk to accept.
Exchanges express that choice as confirmation requirements that differ by asset, and merchants express it by holding low-value payments to fewer confirmations than large ones.
The right threshold is a risk decision rather than a technical constant, which is why two competent operators can reasonably require different waits for the same chain.