For example, F18 is a registered class, but Nacra F18 and Hobie Tiger are also individualy registerd classes. So they are elligble to hold their own Worlds.
That's correct. Nacra and Hobie like to get International status for their own boats so they can hold world championships exclusively for those boats.
An F18 world championship, on the other hand, is open to ALL of the F18 designs.
In other words, if F16 became an International ISAF class, it would not be necessary for the Blade and the Stealth and the Viper, etc. to become International classes. But if the Blade wanted to be able to do one-design world championships, it would have to get International status for itself.
