I know, it's written in such a confusing way. It just means that each slot (which is confusing, I know, because you don't even know how many slots/sub-zones there are in each zone) doesn't have more than one letter in it. That's all. It's like if you had a relative ordering game that stated, "...

We’re allowed to consider the experts, the agricultural scientist and archaelogists he uses, are retarded about their respective fields?