 a service network is defined within the substructure dimension, while the number of service objects and the abstraction levels are defined in the abstraction dimension within each substructure. The substructure dimension in a service network is open ended, i.e. the number of substructure levels is the modeller's choice. In each branch of the hierarchy of substructures, decomposition will stop when the user ports represent services that are available either as reusable library objects or as platform or operating system services. Such services/virtual objects must always be separated from the service network, SO they can be referred to by other service networks. If, e.g. the service S2 in the previous model exists as, or has a potential to be, a reusable component, it will affect the specification of substructures for the virtual object V1 according to Fig. 7.13. Substructure levels A1245 Fig. 7.13 Reusable objects in substructures are referred to by provider port references On substructure level 1, a substructure of two service objects is defined: a vir- tual object V1 and an agent A12. Instead of relating these objects by a service rela- tion, A12 refers V2 by a provider port reference (uS2, meaning: "I am a user of S2"). Further substructuring is then applied only on A12. V2 can now be developed as a service network of its own, which can be instantiated in other virtual objects as well. It is also possible to instantiate V2 as a multiple access object, i.e. an object that is accessed from more than one service network in the sysnet model. 7 Concepts and Notations for Process Control Systems Modelling 7.2.5 Properties The semantic diagram of Fig. 7.14 presents properties that are associated with ports and service objects. defines #*PSid Operation 1. * Operation (see Fig. 7.22) ProviderPort provides defines Primitive Service Resource (see Fig. 7.2) Service Resource relatedBylrn UserPort usesPartial defines Service 1.. (see Fig. 7.2) Behaviour usedForCreating defines Object *) Only agents Primitive Service Behavio