Fluid ounces are not the same as ounces. Fluid ounces measure volume, the amount of space a product takes up. Dry ounces measure weight. The two terms, fluid and dry, are misleading. Fluid ounces can be used to measure dry goods and dry ounces can be used to measure fluids.
People generally use the word ounces when referring to both fluid and dry ounces. The meaning of which measurement should be used is usually clear from the context. For example, when recipes call for an ounce of an ingredient such as sugar, it is asking for volume rather than weight, even though the sugar is dry. A measuring cup is used to measure the correct amount rather than a weight scale.
Some products are always measured and sold by the dry ounce weight. Items such as cereal, sugar and meat products would be difficult to measure and sell by volume, or fluid ounce. Selling those products by weight ensures that each box or package contains the same amount of food.
The Troy ounce is used to measure gold and other precious metals. A Troy ounce is approximately 10 percent heavier than standard dry ounces. For example, an ounce of gold weighs 1.1 dry ounces.