1. Boundary objects – used by actors to communicate with the system. These
include windows, screens, dialogs, and menus.

2. Entity objects – map to objects from your domain model. Includes database
tables and files that hold information that needs to “outlive” use case execution.

3. Control Objects – embody much of the application logic. They serve as the “glue” between the users and stored data (or between boundary objects and entity objects). Controllers are usually not stand-alone classes. They are usually methods that will
be assigned to boundary or entity objects during detailed design.