In the actual cookbooks, which the OP says she's using, there isn't a picture of a cocoa can for each recipe.
Natural unsweetened cocoa is acidic. It is usually paired with baking soda, which is alkaline. The dutching process neutralizes the acidity, making Dutch cocoa neutral pH. It usually gets paired with baking powder, which has an acid added to the baking soda.
Special Dark, which is not exactly the same as extra dark, is a mixture of natural and dutched cocoa powders. This makes it darker than natural, but leaves the acidity somewhere between. If you are baking and using baking soda or powder as a leavener, you need to adjust the acid level accordingly. Since special dark is a mixture, not one or the other, I can't tell you how much to adjust.
The acid level can be adjusted either by changing the ratio of baking powder to baking soda, or by adding an acidic ingredient like buttermilk or lemon juice, or cream of tartar.
By the way, Dutch process cocoa, while darker than natural,is also milder in flavor, not stronger.
In a recipe, if Dutch process is not specified, the default is ALWAYS natural unsweetened.
All of this only matters if you are baking. If you are making candy, ice cream, sauces, etc, use anything you want.