Two Types of Finality

Probabilistic finality

It refers to the type of finality provided by chain-based protocols (eg. Bitcoin’s Nakamoto consensus), in which the probability that a transaction will not be reverted when more blocks are appended after that transaction.

This is why it is recommended to have more than 6 blocks appended to the block that the transaction is included.

How blocks are formed / visual demo

Absolute finality

This refers to the type of finality provided by PBFT-based protocols (eg. DEXON), in which a transaction is immediately considered finalized once it is included in a block and added to the blockchain.

In this case, a leader will propose a block, and a sufficient fraction of a committee of validators will have to approve the block for it be committed.