It is generally understood that the human mind can encompass no more than “5 to 9 different objects.” This precept underlies the principle behind the construction of leveled diagrams in systems analysis and design.
This approach uses an explosion technique instead of an expansion approach. The context diagram is at the top. On it there is usually a single process or transform, labeled “The System,” or something similar. The major external entities to the system and one or two data stores are customarily included, with data flows connecting all the symbols. This diagram is referred to as Level 0. This single process is exploded into its own data flow diagram that reveals the underlying subsystems, which are shown as subprocesses. Each of these subprocesses may, in turn, be exploded into its own data flow diagram to show more detailed processes.
Process explosions continue until the user requirements are completely detailed. Exploding or leveling a system into subsystems and functions is a time-proven technique. Unfortunately, it is frequently done without any real plan, resulting in a collection of illogical subsystems and DFDs that, while somewhat correct, do not seem to make much sense Consequently, you are encouraged to use decomposition files to keep track of the logic of the leveled diagrams: see Decomposition diagrams.