What do sidesystem projects mean when they say "Bitcoin Finality"? Some protocols claim to have “bitcoin finality”. But, for alternative blockchains, bitcoin cannot enforce finality for that system. This means that projects can not directly inherit bitcoin’s double-spend resistance. What projects are saying when they use the term “bitcoin finality” is that their alternative consensus mechanism builds upon checkpoints posted to bitcoin. Bitcoin, in this example, is essentially a bulletin board where protocols post arbitrary data. The interpretation of this data is done by the alternative network of nodes, not bitcoin. The “double-spend resistance” still comes from the alternative network. However, because the network leverages these checkpoints, the network gets some additional benefits that malicious attackers must reorg bitcoin to additionally reorg the alternative network. This is because the networks set up their consensus mechanism in a way that requires block builders to reference the checkpoint posted to bitcoin in a new block proposal. When sidesystem projects say they have “bitcoin finality”, they mean that their own network interprets sets of data that's posted to bitcoin. While checkpointing does provide some security benefits, it should be clear that bitcoin is not the protocol enforcing the rules sidesystem block production. Bitcoin, relative to the state transitions of an alternative networks, is not a settlement layer.