First, a small explanation: there is no term ‘wing’, ‘underbody’ or ‘diffuser’ mentioned in the FIA rules. In the FIA Technical Regulations the area of rear wing is called “Bodywork behind the rear wheel centre line“, area of the front wing is called “Bodywork around the front wheels” and "Front bodywork", and underbody or undertray surfaces are called “Bodywork facing the ground”. There is noting about diffuser either. He is part of “Bodywork facing the ground” rules. You can find, however term ‘Gurney flap’.
For example, front wing is not regulated by wings dimension. Rules are giving you only dimensional imaginary “box” where wings (or anything else) are situated, and same apply for rear wings. Wing elements are called “closed section“ and they have regulated number of them, concave radius and chord. Rear wing assembly, as we know, must have only two elements, main wing element and upper flap element, and that's how they are described in the rules: