When the recipe calls for soy sauce . . .
...but doesn't specify light or dark soy, which should be used? How do I know which soy is better-suited for certain recipes?
Koreans also have light and dark soy sauce.
The lighter soy sauce (gukganjang-국간장) has more of a fermented flavor, is saltier, and used mainly as a soup seasoning. The best soup soy sauce is usually homemade, or made by a producer that only does small batches. A good fish sauce makes a fair substitute.
The dark soy sauce (jinganjang-진간장) is used in stir fries, marinades, fried rice, etc. Kikkoman or similar soy sauces make a fair substitute.
Light Soy Sauce is more commonly used and is saltier, but not to be confused with LITE variations. Dark Soy Sauce is aged longer and has molasses in it. It's more commonly used for making marinades or barbecue sauces. The dark soy sauce may affect the coloring of your dishes. Thick Soy Sauce is also a Dark Soy sauce , but with added cornstarch and sugar....it's used to color dishes and as a dipping/topping sauce for items like Dim Sum.
If they are Japanese recipes, it is regular Japanese soy sauce, which in Japan is also called "koikuchi shoyu" or dark soy sauce. The light colored soy sauce, "usukuchi shoyu", often used for light colored dishes when you don't want to have the dark color of soy sauce, is actually higher in sodium than regular, or dark soy sauce so be careful when using it.
In Japan for low sodium soy sauce, look for "genen shoyu".
This will help if you are looking at Japanese labels:
koikuchi shoyu 濃い口醤油
usukuchi shoyu 薄口醤油
genen shoyu 減塩醤油
As the author of Breath of a Wok let me clarify what I mean by soy sauce. The most common soy sauce used for cooking is what is known as thin or light (not to be confused with low-sodium) because it is the most flavorful and is saltier. Kikkoman makes a great organic soy sauce which I like because it has a clean fresh flavor and only uses organic soybeans, organic wheat, salt, and organic alcohol. In Breath of a Wok I specify dark soy sauce when I want a flavor that is sweeter and richer. Hope that's helpful.