"final" because a conflicting chain of blocks can always outgrow the current canonical chain. However, as blocks are built on top of a transaction, it becomes increasingly costly and thus unlikely for another chain to overtake it. Because competing chains and reorgs can arise naturally, it is recommended that participants wait a number of blocks (i.e. "confirmations") before accepting the probabilistic finality of the transaction. The more confirmations a participant waits, the less risk of encountering a reorg or double-spend.

Double-spending scenarios