Which of the following terms best describes the amount of product in a tank measured from the tanktop to the level of liquid?

The term that best describes the amount of product in a tank measured from the tanktop to the level of liquid is "innage." Innage specifically refers to the vertical measurement of the liquid level within a tank, allowing one to assess how much product is present in the tank against the total capacity. This accurate measurement is crucial for inventory management and safety protocols in handling tanked liquids.

"Thievage," on the other hand, generally refers to the product that is lost through evaporation or other means, rather than measuring the contents of the tank. "Outage" indicates the empty space above the liquid level, which would not provide a direct measurement of the liquid itself. "Ullage" is typically used in reference to the space available or the vapor space in a tank above the liquid level, which again doesn't give a measurement of the liquid itself. Understanding these distinctions helps in managing the contents of a tank efficiently and safely.