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?