What method is used to break down an entire information system into its subsystems?

The method that is effective in breaking down an entire information system into its subsystems is high-level data flow diagrams. These diagrams provide a visual representation of how data flows within a system, illustrating the relationships between various components at a macro level.

By representing the main processes and data stores along with their interactions, high-level data flow diagrams allow stakeholders to quickly understand the major subsystems and how they interconnect. This clarity helps in identifying the boundaries of the system and defines the main functions that need to be further elaborated upon in the design stages.

In contrast, other options serve different purposes. Structured diagrams, for example, may offer a more general framework but do not typically focus solely on data flow. Process specifications delve into the details of specific processes rather than the overarching system layout, and low-level data flow diagrams dig into the details of each process and can become quite intricate, which is not the primary goal when initially decomposing an entire information system.