object may also take inputs in active states. In such cases, the active state is followed by an interrupt transition to a (transient) interrupt state, where the input is han- dled. The behaviour that is associated with a type of provider port is the basic behav- iour described. User-port behaviour is a partial behaviour in relation to the behaviour of the provider port, to which the user port has a service relation. With the exception of interrupt states, the reactive states associated with a pro- vider port are states where the instance value of the service resource model is known to the user. Such states are resource states. The set of all instance values of the service resource model is the service state space. While a provider-port behaviour is in a transition, the corresponding user-port state is a pending state, which implies that the user cannot know in which resource state the provider is. A service object behaviour is a function of the behaviours in all its ports. The states of a service object behaviour are normally all reactive states, either object resource states or pending states. An object resource state represents an instance value of the object resource model. The set of all instance values of the object resource model is the object's state space. Even simple resource models may exhibit thousands of states. Since certain transitions can be allowed in many states, it is important to define subsets of resource states of the state space. Such subsets are EFSM states. EFSM states are defined by pre- and postcondition statements that refer to the state space, i.e. define which part of the state space constitutes the subset. Most state diagrams are built on EFSM states. 7 Concepts and Notations for Process Control Systems Modelling Introduction This chapter presents concepts, notations and suitable information elements for process control system modelling in a more formal way than has been done in pre- vious chapters. The reader should already have a good understanding of the mean- ing of most concepts