As I remember, there is rule says never use "s" for class name. But we have "seats" in business domain and also have "ProductCategories" in Cade's book(p169 class diagram), are we suppose to use such a naming convention?
I think the rule is an 'unwritten' rule (i.e. a best practice). Some people seem to ignore it. I don't think it matters that much, as long as the diagram is understandable.
It may don't matter at high level, but when you look at the 1-n association, or you are going to give the class attributes or operations, people may confused, ex: why we have such a method String getSeatNum() in Seats class?
Don't get me started about those stupid light bulbs.