I'd consider this as having several different possible "levels" of severity:
8 -- If the fit is so bad that the part can't be installed. Why? it fails totally in meeting the requirements of a downstream customer - the assembler. But, nobody is likely to die as a result.
Lower numbers - if the part can be installed, but doesn't meet objective standards. If, for example, there are defined tolerances for gaps on sheet metal. Lower yet if the part subjectively just "looks wrong". Consider the
FMEA manual guidance on whether all customers would b***ch, or just the most discerning.
best of luck..........
Brad