Learn in the textbookRequirements Definition and Design
DefinitionFunctional design (機能設計) is the design phase that decides user-visible specifications, such as screens and forms, from the results of requirements definition; it is also called external design.
ExplanationFunctional design decides the items and layout of the screens users operate, the format of output forms, and the flow of user operations. Because users can see and check these specifications, gaps from what they asked for are easy to spot at this stage.
AnalogyIt is the stage in building a house where floor plans and exterior drawings are made from the owner's wishes. These are drawings the residents can look at to check how livable the house will be.