Form, Fit, Function, and Interface (F3I) (glossary)

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(1) A concept used for the update or upgrade of a system whereby a only a portion or subsystem of the entire system is replaced. The updated item is said to be Form/Fit/Function and interface compatible when it can be inserted into the existing system without impacting system operation. (Created for SEBoK)

(2) A mechanism to link design to performance requirements; i.e., replacing an item/system based on form, fit, function and interface characteristics. (DoD 2004)


(1) This definition was developed for the SEBoK.

(2) DoD. 2004. Guidelines for Engineering, Manufacturing and Maintenance Documentation Requirements for Unique Identification (UID) Implementation, version 1.0. Office of the Principal Deputy Under Secretary of Defense (Acquisition, Technology and Logistics). Accessed on 11 September 2012. Available at


F3I is an important concept regarding the transparency of a change to a portion of a system.

SEBoK v. 2.9, released 20 November 2023