A possibly needless clarification: The DNS is organised by class, name, and then type. Each class is a seperate space, so ycombinator.com in the IN (Internet) class and ycombinator.com in any other class aren't necessarily the same entities. Types can also be class specific, for example A in the IN class is different to A in the CHAOS class. Types may also only be defined in specific classes like SRV (amongst others) in IN. (Aside: this model is why CNAMEs can't coexist with other records.)

There’s also Hesiod (HS) records: https://jpmens.net/2012/06/28/hesiod-a-lightweight-directory...

I'm pretty sure CHAOS just gets used because all the names in class IN have potential meaning to other systems --- you can't just make up random IN names, because you could be screwing up someone else's domain (unless you tediously nest your made-up names under your own domain). But there's no risk of that with CHAOS, so it's a free-for-all for random DNS features.

You'd use a new class, like "RANDOM" or something, except that no deployed DNS software knows about that class.

There's private use ranges for classes and types. They don't have specific mnemonics but you can use the generic ones (eg: CLASS65280 and TYPE65280).

CHAOS probably gets used most because that's what BIND happened to do.

I just sort of assume it's because `dig` already has the string mapped. But maybe people just like typing "chaos".