The way I used arrows might’ve been a bit confusing because I use two types of them.

The solid ones pointing from @alice.com downwards indicate ownership. They’re the same thing as grouping by color. All blue stuff is Alice’s.

The dashed ones pointing between records are links. Those are equivalent of <a href>. Any record can link to any other record, no matter which repositories either is in.

When you comment on someone’s post, your comment goes into your repo, but it has a link to the parent post (which may be in any repo). That’s usually how you want to represent it in the data model so that anyone indexing both records can reconstruct the relationship.

In the example, Bob comments on Alice’s post. So Bob’s comment is in Bob’s repo and Alice’s post is in Alice’s repo.

To clarify your specific point, a person commenting on your post will create record in their repo. In fact one can never create records in somebody else’s repo. That’s the central premise.

Hope that makes sense.