Apparently a boost/repost is kind 6, which I can't find mention of in the NIPs? And snort includes the reposted event in full as stringified JSON in the content field, while iris leaves the content field blank?
Discussion
So, kind 6 was NIP-18, but it got removed because it wasn't being implemented as specified: https://github.com/nostr-protocol/nips/issues/173
Quoting works by regular text notes with mentions of other notes embedded in the text body. I would have assumed reposts were the same but with no other text in the body.