With the fanout pattern, messages are delivered to one or multiple destinations (that is, listening clients or subscribers) through a single message queue. Subscribers retrieve the same message from the queue, rather than their own unique copy. While this can improve performance, it also makes it more difficult to verify whether or not a subscriber received a message.
This is a Level 3 Documentation and Implementation diagram according to the Salesforce diagram framework.
Learn more by reading the Architect's Guide to Event-Driven Architecture with Salesforce.