Most blockchain topologies today have probabilistic finality, in that you are never 100% sure whether or not the transactions are truly set in stone. For example, in BTC, you get an exponential falloff in the probability that an attacker will be able to catch up to the rest of the network and reorganize the blocks as time (or in the world of BTC, # of blocks) goes forward. This exponential risk falloff gave rise to the “6 block” rule of thumb whereby once you’ve observed 5 blocks built on top of the block containing your transaction, making your transaction “6 blocks deep”, then it is statistically highly unlikely that your transaction would be part of some reordering attack.