In the conversation model, all comments are sent to and re-delivered by the "owner" of the top-level post in the conversation. That's in quotes because the owner isn't necessarily the author. In fact it is the 'sender'. The owner also sets the privacy for the conversation, and can remove comments from the conversation or delete it completely. It belongs to them. The mechanism supports things like private groups and 'circles/aspects' because the conversation owner is the authority of who was addressed in this conversation. They are the only entity that knows specifically who was addressed, which may be private lists they control and which nobody else has permission to enumerate. If they relay all the comments, the conversation remains complete at all nodes that received the top-level post.

In practise, (and here is where we differ significantly from the 'posts' model) -- you still have threaded conversations, and you still specify the inReplyTo field to indicate which comment you are responding to, but you still must send the post to the conversation owner for delivery -- who delivers it to everybody in the original conversation audience accordingly. You cannot change the privacy to something else or inject DMs in the middle of the thread. The conversation does not belong to you or the person whose post you're commenting on. It belongs completely to the sender of the first post in the thread. What's interesting is that Diaspora and Friendica developed the exact same mechanism simultaneously and independently for supporting conversation privacy.