https://github.com/nostr-protocol/nips/blob/master/29.md
It's the two examples given in the NIP so far.
(though, I don't really get yet why it's not just kind 1's for post for example)
But as far as I'm concenrned it's a different use case.

Chat = "Gm frens", chronological chaotic feed, fun and giggles
Post / Thread = "Here's my thought I'd like to have an more depth conversation around) 

 Thanks, that nip is enormous, need to think it through