In its original proposition (Gero, 1990), the FBS model identifies eight elementary processes of a design activity that are described by three types of variables: ‘Function’, ‘Behaviour’ and ‘Structure’.
Design is described as an activity that transforms requirements into design descriptions, while the requirements are
considered equivalent to functions embodying ‘the expectations of the purposes of the resulting artefact’. The first process is called formulation and transforms functions F into a description of a behaviour that an artefact is expected
to perform to meet the previous functions.