Formula rules can limit building materials also .Box rules do not.
A and C class cats are box rules- You can use anything to build them.Sky is the limit.
The F-18s are formula because of material limitations( aluminum stick, No carbon, etc). I can't speak for F-16 because I never read their rules.
At least that's my take on it.
OK...."Formula" is synonymous with "ingredients" not measurement technique. Formula classes can also employ a box rule, formula, or both for its measurements. 12-Meter keel boats do have materials ("ingredients") limitations for its hulls. So far that works. Looked at the A-Cat ISAF class rules, 3 pages. Page 1 is just a cover page and page 3 are just a few lines. No materials restrictions. Love it!
