Basically when a user publish a message to a channel, on the comet server, the server will broadcast the message to all the subscribers of that channel. In most cases, the author of this message is usually a subscriber of that channel also.
In this case, let’s assume that the publisher is also a subscribe of that channel, in normal cases, the publisher would get a copy of the message, but we could tweak it slightly that now the publisher no longer receive its own messages.
In this case, there might be cases where we want to find out some information about a channel but we don’t want other subscribers to receive it. In most cases, this is due to security reasons where you don’t want other subscribers to know certain things, or it could be to prevent unnecessary flooding. Of course in this picture, the diagram should be interpreted slightly differently, instead of treating the blue arrow as the same copy of the message from the red arrow, the blue arrow should be more relevant information provided by the server. One such information could be the list of subscribers to that channel. Due to the nature of such information, it would not be desirable to broadcast the list to everybody everytime any of the subscribe make such a request.
In this case, we are trying to simulate P2P communication, where a subscriber can talk to another subscriber.