Joining your "local" country sailing association, like US Sailing here in the US, has nothing whatsoever to do with getting International Class status with ISAF.

I don't know why the F16 Class would want to become an International Class under ISAF rules, because then the class basically loses control over its own class, and every little change it might want to make in its rules has to be approved by ISAF. And manufacturers of that boat have to adhere to very strict standards as established and approved at the time the boat first becomes an International Class (unless you can get ISAF to approve a variation).
And in order to become an International Class you have to have X number of boats racing actively in X number of countries on X number of continents.

I haven't been able to figure out why any class would want to be an ISAF International Class except for the fact that ISAF will not allow a class to hold a World Championship if it is NOT an approved International Class.

ISAF claims to control the words "World" and "International" and anything that implies that something is of this world.

It is because of ISAF that the F16 Class is not able to have an International Class Association. That is not allowed by ISAF, since the F16 is not an International Class. That is why the F16 Class has a "Governing Council," to get around that "international" thing.

And that is why the F16's can't have a world championship and have to call it something else.

End of rant.