Design patterns·Modelling the domain
the operation genuinely belonged to no single object — it needed two of them — so it became an object of its own.
Domain service
Also calleddomain service object, stateless domain operation
A stateless object holding a piece of domain logic that does not naturally belong to any entity or value object, such as a transfer between two accounts. It is legitimate and deliberately rare: the more of them a model has, the more likely the entities have been hollowed out. The test is whether the operation would be arbitrary to place on either participant.