are there any guidelines on when you should put an actor also on a domain model ? By that I mean should you put things like Customer, Supplier, Employee which are actually users/actors for the system under design, also on the domain model. Maybe more interestingly, when should you not do this ?
Sun Certified Developer for the Java 2 Platform
Sun Certified Enterprise Architect for the Java Platform, Enterprise Edition 5
In addition to Kengkaj's advice, I think "Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and Iterative Development (3rd Edition) - Craig Larman " will help to enhance understanding.