Way back when I worked for the Defense Logistic Agency (1979) we were told we could no longer use the term "Form, Fit and Function" as it was too ambiguous. We had to use the phrase “Interchangeable as defined by MIL-STD-280A” .
from MIL-STD-280A
Interchangeable item .- One, which (1) possesses such functional and physical characteristics as to be equivalent in performance, reliability, and maintainability, to another item of similar or Identical purpose; and (2) is capable of being exchanged for the other item (a) without selection for fit or performance, and (b) without alteration of the items themselves or of adjoining items, except for normal adjustment.