With the publish / subscribe (unique copy) pattern, publishers and subscribers exchange messages via one or more channels in an event bus. Subscribers listen to the appropriate channels and receive new messages as they arrive. Unique copies of each message are sent to each subscriber, which makes it possible to guarantee delivery and identify which subscribers received which messages. Systems can also use replay functionality to recover past events, which helps to ensure resilience against system failures.
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.