What do block diagrams primarily illustrate?

Block diagrams primarily illustrate functional units and their relationships within a system. This type of diagram provides a simplified representation of complex systems, where various components or processes are represented as blocks connected by lines that indicate the relationships and flow of information or control between them. By highlighting these functional relationships, block diagrams help technicians and engineers understand how different parts of a system interact and function together, which is crucial for troubleshooting, system design, and analysis.

In contrast, the other options focus on different aspects: the physical layout relates to spatial arrangements of components, maintenance procedures are about operational practices for upkeep, and energy consumption deals with the metrics of resource usage rather than the relational aspects of system components. Thus, the focus of block diagrams is specifically on functionality, making them an essential tool in systems engineering and electronics.