ction hierarchy (Sect. 2.3.1) The dualism between interface and unit (Sect. 2.3.2) Interface refinement (Sect. 2.3.3) Substructures (Sect. 2.3.4) Interface transformation (2.3.5) Functional space (Sect. 2.3.6) Integration hierarchy (Sect. 2.3.7) While these concepts and principles are described in this section, their applica- tion is discussed in detail in later chapters. 2.3.1 Abstraction Hierarchy The resources and behaviour of a processing system are specific for the type of sys- tem. The structure of the process control system depends on the complexity of the resource system and on the physical distribution of resources. The method for mod- elling a process control system is, however, the same for all processing systems. To handle a specific resource system, however, a process control system must have knowledge about every controllable property of the resource system. This knowledge makes a process control system dedicated to a particular processing system. The knowledge is stored in and manipulated by the process control system as information. This information can be described in a model of the resource sys- tem, called the resource system model. A resource model is a kind of conceptual model. Since the process control system is a structure of entities, the resource sys- tem model is partitioned accordingly. When we want to be precise, we will refer to partitions of a resource system model as resource model fragments. Figure 2.13 shows the principle for creating a dedicated process control system. 2 Foundations of Sysnet Modelling Resource System Process Control System Sensor Resource model of Y Process Effector controller Resource controllers model of {X,Y} Sensor Resource model of X Effector models is a _partition_o resource system model Data note. The AML symbol for a data element. In this figure it symbolizes resource models. Fig. 2.13 A dedicated process control system The way a resource system model is partitioned depends on the depth of the con- trol structure (which is a design decision), and compris