In fanout pattern, many interested parties will bind (a.k.a, subscribe) to a given topic.
When a message is sent to the topic, all subscribers will receive a copy of the message sent to the topic. The message is “fanned out.”
So ROS calls itself Pub-Sub when really it is fanout.
Twitter is a good example of the Fanout pattern. One single tweet is sent to all the parties following the person sending the tweet.