What are entities?
An entity is a documented business object or concept that your architecture cares about.
Examples of entities include Order, Customer, Invoice, Payment, Shipment Product.
Entities are useful when you want to describe the shape and relationships of your domain model, separate from the services, messages, and systems that use it. For example, in an event-driven system, OrderCreated might be the message, OrderService might be the producer, and Order is the entity that gives the event business meaning.
Entities can be referenced from Domains, Systems and Services.