For the purposes of energy statistics, stocks are quantities of energy products that can be held and used to: (a) maintain service under conditions where supply and demand are variable in their timing or amount due to normal market fluctuations, or (b) supplement supply in the case of a supply disruption. Stocks used to manage a supply disruption may be called ”strategic” or “emergency” stocks and are often held separately from stocks designed to meet normal market fluctuations. A stock build is shown as a negative number, and a stock draw as a positive number. They are calculated as a difference between the closing and opening stocks.