Within the context of a project, we describe the logical behaviour and information needs of a putative system to test and clarify the requirements. We specify interfaces and define the realisation of use cases within a system including state based and state independent dynamic behaviour. The process of developing such models refines the requirement and drives out ambiguity. The result is a specification that can be used by developers to build the right system just as a circuit diagram describes how an electronic system will behave. A key benefit of modelling systems at scale is the support it provides to systems integration. We support successful integration by developing and maintaining specifications of the information exchange requirements and dynamic behaviour between interacting systems.