Embedded computer control systems are a kind of safety critical real-time system, formed by a synergetic integration of various functions, technologies, and engineering disciplines. To modularize the design and to define the embodiments of functional modules in terms of software components, a more precise modeling of systems and support for assessments of modularity and related tradeoffs in designs is necessary. However, little support exists in this area. Existing system models and modularity metrics such as coupling and cohesion are often either too general or too specific by targeting only a specific implementation technology. This paper proposes a meta-level model for the systems and a fine-grained classification of relationship patterns established by communication, synchronization, and implementation. The work emphasizes a system perspective and aims to support a more precise assessment of coupling, considered as one of the most important criterion for component creation and integration as well as structuring.