Are you talking about the rating system for whitewater?

Class 1 (I) Easy: Very little whitewater.

Class 2 (II) Moderate: Some whitewater. Small amount of maneuvering required.

Class 3 (III) Intermediate: More whitewater. Longer rapids requiring some precise maneuvering.

Class 4 (IV) Difficult: Long, continuous rapids with difficult, precise maneuvering required. Generally scouted before running.

Class 5 (V) Very Difficult: Extremely difficult maneuvering required. Very dangerous. High probability of capsizing. Very rare on commercial trips.

Class 6 (VI) Not Navigable: Unrunnable.