System Design Document. A document that describes and defines the system (or high level) design of the product. There are five main portions of this document: archtecture of the project, high level design of the product, including the class diagram and class specifications, validating the design, how th product will interface with other systems and the Build Plan. This plan shows how the project will be incrmentally implemented in phases. Each phase will deploy one or more use-cases that QA and Testing will verify. Software developers typically develop this document.