the categories were set up for charter & sail training boats and define the waters they are allowed to sail in for those purposes. the categories are decided by angle of vanishing stability. i can't remember the exact figures but it's something like this:
0: unlimited, ocean going
1: within 150 miles of safe harbour
2: within 60 miles
3: within 10 miles
4: inland & sheltered coastal waters
don't take these for gospel, i'm sure someone can give the exact figures.
i particularly seem to remember that you need at least cat 2 for channel crossing and cat 1 is good for anything except ocean crossings.
Current French categories are defined as follows .... which appear somewhat different - notably no category 0. I wonder whether you were using ORC Special Regs categories, rather than the French Categories de navgation which are applied to all French boats and French skippers in French territorial waters?
1ère catégorie : navigation n'entrant pas dans une des catégories ci-dessous.
2ème catégorie : navigation au cours de laquelle le navire ne s'éloigne pas de plus de 200 milles d'un abri.
3ème catégorie : navigation au cours de laquelle le navire ne s'éloigne pas de plus de 60 milles d'un abri.
4ème catégorie : navigation au cours de laquelle le navire ne s'éloigne pas de plus de 20 milles d'un abri.
5ème catégorie : navigation au cours de laquelle le navire ne s'éloigne pas de plus de 5 milles d'un abri.
6ème catégorie : navigation au cours de laquelle le navire ne s'éloigne pas de plus de 2 milles d'un abri.
I think this means that it is not *too* dangerous and you can visit it once a month, but it won't be allowed out until after the first five years, with good behaviour.