user2306: Some people have suggested repurposing OP_NOP3 as a new opcode, OP_CHECKSEQUENCEVERIFY.
What would the new opcode do? Why is it useful? Can't the signer choose the sequence field when they sign?

user3697: OP_CHECKSEQUENCEVERIFY (a.k.a OP_RELATIVECHECKLOCKTIMEVERIFY) can do relative lock times (used for sidechains and lightning).
It basically checks, if an input's sequence number is smaller than the sequence threshold (1 << 31 = 2147483648), and if so, it will compare the nLockTime against the sequence number of the input.
More info:

https://github.com/ElementsProject/elementsproject.github.io/tree/master#relative-lock-time
https://github.com/maaku/bitcoin/commit/fd9864c3a3c6f2942bc0775c6c725aa379f9d616
https://lightning.network/lightning-network-paper.pdf


2306: <p>Some people have suggested repurposing OP_NOP3 as a new opcode, OP_CHECKSEQUENCEVERIFY.</p>

<p>What would the new opcode do? Why is it useful? Can't the signer choose the sequence field when they sign?</p>
 3697: <p><code>OP_CHECKSEQUENCEVERIFY</code> (a.k.a <code>OP_RELATIVECHECKLOCKTIMEVERIFY</code>) can do <strong>relative lock times</strong> (used for sidechains and lightning).</p>
<p>It basically checks, if an input's sequence number is smaller than the sequence threshold (1 &lt;&lt; 31 = 2147483648), and if so, it will compare the <code>nLockTime</code> against the sequence number of the input.</p>
<p>More info:</p>
<ul>
<li><a href="https://github.com/ElementsProject/elementsproject.github.io/tree/master#relative-lock-time" rel="nofollow noreferrer">https://github.com/ElementsProject/elementsproject.github.io/tree/master#relative-lock-time</a></li>
<li><a href="https://github.com/maaku/bitcoin/commit/fd9864c3a3c6f2942bc0775c6c725aa379f9d616" rel="nofollow noreferrer">https://github.com/maaku/bitcoin/commit/fd9864c3a3c6f2942bc0775c6c725aa379f9d616</a></li>
<li><a href="https://lightning.network/lightning-network-paper.pdf" rel="nofollow noreferrer">https://lightning.network/lightning-network-paper.pdf</a></li>
</ul>
