They are all edible. It's just the percentage of cocoa content versus other ingredients (primarily added sugar). Most brands have this clearly marked on the wrapper. Once you get above about 70% you get a significantly bitter flavor, but with the growing popularity of eating dark chocolates, you'll find people eating what used to be solely qualified as "baking chocolate." There are some people who enjoy eating 90% or higher which is really bitter.
Milk chocolates are usually in the 30s-40s percentage range, dark chocolate is about 50% and up. It used to be that 70% or higher was for baking, but designer chocolates have made it trendy to eat those.