What is "archetype" field in latest.json?

What is the purpose of “archetype” field in latest.json?

There’s three types of archetypes in Discourse by default.

and they refer to the type of the topic.

regular: it’s just a normal topic like the one you made here.
private_message: the topic is a private message
banner: the topic has been marked as a global banner.

In latest.json you will only see archetype: "banner" if the topic in that list has been marked as a banner. Everything else will be regular because that list does not include any PMs.