What Is the Significance of the ‘Nsequence’ Field in Enabling RBF?

The 'nSequence' field is a four-byte field in each transaction input that, when set to a value lower than 0xFFFFFFFE, signals the transaction is eligible for replacement under the BIP125 Opt-in RBF policy. Its original purpose was for relative time-locks, but it was repurposed to allow the sender to signal their intent to potentially replace the transaction later.

If the field is set to the maximum value (0xFFFFFFFF), the transaction is considered final and cannot be replaced under the opt-in rules.