The realisation relationship links a logical entity with a more concrete entity that realises it.
The realisation relationship indicates how logical entities ("what"), such as services, are realised by means of more concrete entities ("how").
A business process or function may realise a service. A data object may realise a business object. An artifact may realise an application component.
Structural.
An application component, "Financial application", realises a "Billing" service.
A "Billing" data object realises the business object, "Invoice".