On top of the topological ontology the standard system-theoretic notions such as system, subsystem, system boundary, environment, open/closedness etcetera can be defined. Some of these definitions can be found in Figure 5. The ontology projection is of the include and extend type, just like in the topological ontology.
Figure 5: Excerpt from the systems theory ontology. This ontology introduces system-theoretic notions on top of the topological ontology.
Definition 3 states that a system is a mereological individual (but not every mereological individual is a system). The in-system(x,s) holds for individuals that are in the system and are not subsystems of it. This is different from the subsystem-of(sub,sup), where the part must be a system. A connection is in the boundary of a system when it connects an individual in the system to an individual outside the system. With this definition, the classes open-system and closed-system can be defined easily.