"As an Administrator, I want to assign a Client Developer to a rate limit class, so that the developer has the rate limit of that class."
User story 8 implies that it should be possible to assign client IDs to a class; this makes it explicit. I think this would be 1:many; a client ID would be a member of exactly one class. I don't believe it makes sense to be a member of more than one class (which limit would you use?) or zero classes.