It's a document or a type approval issued by the builder stating the seaworthiness of the boat. I'm not sure whether this document is needed all over the world but you need in France in order to register the boat on your name.
In this case the document would state that the catamaran is a class D: seaworthy for coastal sailing up to 2 miles with winds up to 6Bf and swell up to 2 metres.
Don't know if the correct English term is 'certificate of conformity' but hope that above gives a better idea of what I'm looking for. Thanks for the help (and your question).