While most 100% dark chocolates are plain, some are flavored with an inclusion or sprinkled with extra cacao nibs, sometimes in addition to lecithin. But the resultant chocolate is still considered 100% dark chocolate, because the reference of “100%” is towards the lack of sugar more so than the purity of a single ingredient. The percentage by weight of cacao in a 100% dark chocolate bar should still be over 99%. Any larger inclusions— nuts, berries, salt, herbs, etc.— are added apart from the chocolate itself, such that the chocolate surrounding those additions is still 100% dark chocolate.
If a chocolate contains alternative sugars, such as stevia, xylitol, or monk fruit extract, it cannot be called a 100% chocolate, though it’s often labeled as sugar free chocolate. You may also see "chocolate" as an ingredient on a chocolate bar. That's a reference to the ingredient of cacao mass, the ground-up cacao which becomes chocolate, and it should always be the first ingredient in a dark chocolate. If a product is advertised as 100% dark chocolate, then it should be made of 100% cacao and precious little else.