I believe it was originally defined in nostr:npub180cvv07tjdrrgpa0j7j7tmnyl2yr6yr7l8j4s3evf6u64th6gkwsyjh6w6's web commenting nip, then clients started to add it to any note when it contains a link, including damus.

So if someone links to some article, a browser plugin that pulls notes for that article would see your comment from nostr.