I use organic dark chocolate that is made by a different company that has a factory with two machines. One is for dark chocolate (with vegan ingredients) and the other is for milk chocolate. The only thing connecting these machines is the air in the factory, otherwise they are completely independent of each other. The problem is that the milk chocolate machine uses a very fine milk powder and some of these milk powder particles can potentially be caught in the air currents. This milk powder MAY find its way into the dark chocolate machine. The company tells me that if this was to happen it would be at most a few parts per million. Because this event could happen the chocolate has to be labeled as "may contain milk".
Sometimes I use nuts in the chocolate preparation area. This does not happen often but if you do suffer from an extreme nut allergy please avoid eating the chocolates.