Opt-in Full Replace-by-Fee Signaling.

Lets a sender mark a payment as replaceable, so it can be swapped for one paying a higher fee if it is slow to confirm.

Many nodes wouldn't replace a waiting payment with another that spends the same coins, so a stuck payment was hard to fix. With this signal, nodes may accept a replacement. A recipient may choose to wait for a confirmation before treating a payment that carries the signal as paid.