[sailingdog]

Actually, there are four categories, here they are with their descriptions:

A. Ocean: Designed for extended voyages where conditions may exceed wind force 8 on the Beaufort Scale and significant wave heights of 4 meters and above, and vessels are largely self-sufficient.

B. Offshore: Designed for offshore voyages where conditions up to, and including, wind force 8 and significant wave heights up to, and including, 4m may be experienced.

C. Inshore: Designed for voyages in coastal waters, large bays, estuaries, lakes and rivers where conditions up to, and including, wind force 6 and significant wave heights up to, and including 2 m may be experienced.

D. Sheltered waters: Designed for voyages on small lakes, rivers and canals where conditions up to, and including, wind force 4 and significant wave heights up to, and including, 0.5 meters may be experienced.

Significant wave height is considered to be the primary factor and other parameters are descriptions of when these wave heights may be expected to occur. Significant wave height means the average height of the highest 1/3rd of the waves over a given period. Waves of double that height may occasionally be experienced.

It is also possible for a given boat to be assigned to more than one design category with different maximum capacities corresponding to each category. For instance, a boat may be EU RCD B with four passengers, but only EU RCD C with six people aboard.

Be aware, even though a boat may have an EU RCD category of A doesn't necessarily mean it is suitable for making bluewater passages. In many ways these are minimum, baseline standards, and not recommendations for suitability of purpose.