"Subsystems" Are Actually Requirement Packages

Common Mistakes 01

We often hear statements like, "This system is divided into eight subsystems, including the sales subsystem, financial subsystem, inventory subsystem..." This is an example of not distinguishing between requirements and design. In fact, the speaker might mean to say, "The functional requirements of this system are divided into eight requirement packages..."

Requirement packages are based on the stakeholder's perspective to partition the system's functions, whereas subsystems (components in UML terms) are based on the internal perspective to partition according to the coupling and cohesion of system parts. These two are not one-to-one correspondences. The so-called "financial subsystem" might actually be "putting the functions used by financial personnel into one package," as shown in Figure 1-2.