Modules are organized in a hierarchical manner. For instance, you may define a module to handle a pump that also contains child modules to handle tasks for the pump such as drawing it on the screen, handling operator interaction, doing the control logic, handling the electrical and hydraulic aspects of the pump, etc.
With this parent-child relationship, you can subdivide the application into components of a manageable size. Each child module may have its own child modules as well. This also facilitates working on the application in a team environment since each module is a separate, self-contained portion of the entire system. Any child modules defined within one module do not affect the operation of any other module in the system.