What does "top third" mean?
Hello, would you please tell me what "top third" mean in the following sentence?
"Approved slaughter plants normally grading after one Business Day must hold carcasses falling in the top third of any quality grade except Prime for re-grading two Business Days after slaughter."
Does it mean one third of the top of any grade of carcasses except Prime, or does it mean the top three grades of carcasses among all grades of carcasses except Prime, like Choice, Select and Standard?
Thank you very much :)

Question

Answer

If you divide something into three, you produce three thirds.  You can classify these thirds as the top third, the middle third, and the bottom third.

In this example, there are a number of quality grades for carcasses. So each carcass is put into a particular grade.

Your text does not say it, but we must assume that there is another procedure for grading carcasses within each grade, and this process determines the best or top 33.3% of the carcasses in each grade.  The text then tells about a specific rule which applies to the top third of carcasses in each quality grade (except Prime).