user51948: I've seen various 'options' that the different DNS seeders support, for example x1, x5, x9, and xd:
vSeeds.emplace_back("seed.bitcoin.sipa.be", true); // Pieter Wuille, only supports x1, x5, x9, and xd
vSeeds.emplace_back("dnsseed.bluematt.me", true); // Matt Corallo, only supports x9
vSeeds.emplace_back("dnsseed.bitcoin.dashjr.org", false); // Luke Dashjr
vSeeds.emplace_back("seed.bitcoinstats.com", true); // Christian Decker, supports x1 - xf
vSeeds.emplace_back("seed.bitcoin.jonasschnelli.ch", true); // Jonas Schnelli, only supports x1, x5, x9, and xd
vSeeds.emplace_back("seed.btc.petertodd.org", true); // Peter Todd, only supports x1, x5, x9, and xd

(from chainparams.cpp)
But I can't find anything which actually explains what these options are. What do these options do and why do only some seeders offer some of the options?

user2306: This is 'service bit filtering.'
You can run nslookup seed.bitcoin.sipa.be, and get a list of bitcoin nodes.
You can also run nslookup x1.seed.bitcoin.sipa.be, and get a list of nodes with NODE_NETWORK set.
(source 1. source 2.)

51948: <p>I've seen various 'options' that the different DNS seeders support, for example x1, x5, x9, and xd:</p>

<pre><code>vSeeds.emplace_back("seed.bitcoin.sipa.be", true); // Pieter Wuille, only supports x1, x5, x9, and xd
vSeeds.emplace_back("dnsseed.bluematt.me", true); // Matt Corallo, only supports x9
vSeeds.emplace_back("dnsseed.bitcoin.dashjr.org", false); // Luke Dashjr
vSeeds.emplace_back("seed.bitcoinstats.com", true); // Christian Decker, supports x1 - xf
vSeeds.emplace_back("seed.bitcoin.jonasschnelli.ch", true); // Jonas Schnelli, only supports x1, x5, x9, and xd
vSeeds.emplace_back("seed.btc.petertodd.org", true); // Peter Todd, only supports x1, x5, x9, and xd
</code></pre>

<p>(from <a href="https://github.com/bitcoin/bitcoin/blob/master/src/chainparams.cpp" rel="nofollow noreferrer">chainparams.cpp</a>)</p>

<p>But I can't find anything which actually explains what these options are. What do these options do and why do only some seeders offer some of the options?</p>
 2306: <p>This is 'service bit filtering.'</p>

<p>You can run <code>nslookup seed.bitcoin.sipa.be</code>, and get a list of bitcoin nodes.</p>

<p>You can also run <code>nslookup x1.seed.bitcoin.sipa.be</code>, and get a list of nodes with NODE_NETWORK set.</p>

<p>(<a href="https://bitcointalk.org/index.php?topic=1888573.msg18791989#msg18791989" rel="noreferrer">source 1</a>. <a href="https://github.com/bitcoin/bitcoin/blob/9d31ed2e69fd5cde5fbc5208bd04276a58afd132/src/net.cpp#L1578" rel="noreferrer">source 2</a>.)</p>
