Isn't it true that 16 oz of dry goods is different than 16 oz of liquid measure.  If a recipe calls for 8oz of flour, you should measure it using a scale rather than using a cup (which is a liquid measure, no matter what kind of cup you use).  I'm confused.

--SM

The confusion comes from the fact that in English we use the word "ounce" when talking about both the weight of something or the volume  it occupies.  In cooking, if you are talking about the volume of something, technically you should refer to the measure as "fluid ounces" and if  you are weighing something then you can say just "ounces."

Ingredients such as beans, sugar or flour can be conveniently measured by volume in cups, pints, quarts, liters and all of the other units that are used to calculate volume, including fluid ounces.  They can also be weighed in pounds, kilograms and ounces.

Depending on what ingredient you are measuring, the weight in ounces and the volume in fluid ounces may be close to the same or vastly different.  Sixteen fluid ounces of water weigh very close to 16 ounces (1 pound)^*.

Sixteen fluid ounces (2 cups) of flour will weigh around 8 ounces (1/2 pound) if it is sifted, and somewhat more, around 9 ounces, if it is unsifted.  This is just a rough estimate, though.

So, if your recipe calls for "fluid ounces" then you are definitely using a volume measuring device, such as a liquid or dry measure.  See the post Difference Between Dry and Liquid Measuring Cups for guidance on which to use and how.

If the recipe just says "ounces", for some ingredients you may be able to assume that the author means to measure by weight.  For ingredients like flour, though, the author may mean either weight or volume so you need to be careful to understand which is being used.  In books on baking there is frequently a section near the start on techniques where sometimes they may clarify which measure they are using.

As a further note, fluid ounces may be abbreviated in recipes as "fl. oz." and ounces as just "oz."

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^* The scientist in me wants to say that is true provided that the water is close to room temperature and the ambient air pressure is close to standard pressure at sea level, but for most practical purposes you can consider them to be equivalent.