That just provides a "this exists and is a thing" overview, with the bulk of the information hiding in https://tools.ietf.org/html/rfc6761.

That's the link that qualifies special behavior for anything ending in ".test", ".localhost", ".invalid", and the set of "example.???" domains.

Copy-pasting the RFC into a comment would be a bit spammy (it's three pages of hyper-specificity), so just go read that. It's quite accessible and the mechanics are useful to be aware of.