Concept from Martin Fowler's Patterns of Enterprise Application Architecture. Defines the application boundary and the set of available operations, coordinates domain-object responses, and controls transactions inside those operations. Services in the service layer encapsulate application business logic. Fowler allows them to be called remotely or locally; Ivan argues the layer is most meaningful in distributed/remote contexts and can often be ditched in a plain Rails monolith. Fowler does NOT use the term 'service object' — this is the closest concept in his book.