are unidirectional. A relation name has an initial lowercase letter and should be defined SO that the relation can be read as a natural language sentence, see Fig. 7.17. controls Interpret as: "A controls B" isControlledBy Interpret as: "B is controlled by A" Fig. 7.17 Names of relations Relations can be annotated with cardinallity information, se Fig. 7.18. controls Interpret as: "A controls exactly one B" controls Interpret as: "A controls exactly L Bs" (L=2,3,...) controls Interpret as: "A controls K or more Bs" (K=0,1,...) controls Interpret as: "A controls from K to M Bs" (M=1,2,...) controls K..M,N B Interpret as: "A controls from K to M or N Bs" (N=1,2,...) Fig. 7.18 Cardinallity notations The property list is associated with the resource by a single relation. Properties in the list are separated by an asterisk (*). Properties are written as identifiers, which means they are names that have initial lowercase letters. A property can also be qualified as used for identification (#), as being optional (o) or/and as being a generator of internal initiating events (iie), see Fig. 7.19. Switch # *swld o *channelLimit *qosData iie *faultState Fig. 7.19 The property list The resource Switch is described by four properties. The property SWid identifies a switch. The property channeLlimit is optional, i.e. a Switch can exist without having any defined channeLlimit. The mandatory property qosData (quality of service parameters). The property faultState is a trigger property for internal initiating events. This means that when this property gets a specific value, or a value within a specified value range, some predefined activity must start in the corresponding object 7 Concepts and Notations for Process Control Systems Modelling (without any user request). The value that can trigger such an activity should be defined in the property specification part, see Sect. 7.3.4. 7.3.3 Rules Some relations are simple relations between instances, based on cardinallity information such as the "A controls exactly one B" relation. 