What is the highest class diamond?

Could you clarify for me what exactly is meant by the term "highest class diamond"? Is it referring to the highest quality diamond in terms of clarity, color, cut, and carat weight? Or is it perhaps a reference to a specific, prestigious classification system for diamonds? Additionally, I'm curious about the factors that contribute to a diamond being considered of the highest class. Is it solely based on its physical attributes, or are there other factors, such as rarity or historical significance, that come into play? I would appreciate a comprehensive description of the criteria used to determine the highest class diamond.