Are System unit system cabinet and chassis interchangeable terms?

Answer 1

No, not usually. A chassis is a major subassembly, usually purely mechanical, that supports component parts. A system cabinet is a box containing one or more chassis with provision for connections between one chassis and another, and with the outside world. It usually has one or more display units giving information to the operators, and may have provision for input of information or instructions. A system unit is difficult to explain because there is no general agreement among engineers as to what exactly it is. The term originates from sales and marketing departments more often than from the designers. An engineer would probably say (if asked !) that a system unit is one or more chassis enclosed in a system cabinet.