Form, Fit, and Function (also F3 or FFF) is a concept used in various industries, including manufacturing, engineering, and architecture, to describe aspects of a product's design, performance, and compliance to a specification.
F3 originated in military logistics to describe interchangeable parts: if F3 for two components have the same set of characteristics, i.e. they have the same shape or form, same connections or fit, and perform the same function, they can be substituted one for another.
In practice, F3 is usually used not for final products (like entire weapon systems), but for the procurement of components and subsystems.
[2] FFF refers to a set of characteristics or requirements that are essential for the design and compatibility of products, components, or systems, and can have legal considerations in regulated industries like aviation and defense (e.g., for technical data rights and configuration management).
[6] The United States (US) Government formally recognized it in the legal incorporation of Public Law 98-525 regarding technical data and design changes.