Form, Fit, Function, and Interface (F3I) (glossary)
(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.
(2) is a mechanism to link design to performance requirements, i.e., replacing an item/system based on form, fit, function and interface characteristics.
(2) Guidelines for Engineering, Manufacturing and Maintenance Documentation Requirements for Unique Identification (UID) Implementation. www.acq.osd.mil/dpap/Docs/uid/UIDEngDocGuidever4_120604.doc
F3I is an important concept regarding the transparency of a change to a portion of a system.