Shrimp are sold by size and the standard quantitative unit in the United States is count per pound. For instance "16/20" denotes 16-20 shrimp per pound. This method is valid for fresh or frozen in the shell without the head on.
The "U" designation stands for "under", meaing less-than the count per pound. For instance, "U10" means that the shrimp are large enough that less than 10 make up 1 pound. To help the consumer to determine the size, a trade designation such as Jumbo or Medium, may accompany the count per pound on retail packaging.