The process of chocolate making involves roasting, peeling, and refining cacao beans into a smooth mass. A bar of 100% dark chocolate is made with zero sweetener, and often no ingredient other than cacao beans. Some companies use additional cocoa butter or a small amount of plant lecithin to smooth out the chocolate in the refiner, but must maintain that chocolate at a minimum of 99.75% cacao by volume.
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.