A causal link is a link from a chance node (discrete or continuous) or a decision node to a chance node.
It defines a causal dependence from the parent node to the child node. That is, the state of the parent has an impact on the state of the child.
It can sometimes be difficult to determine what is a causal dependence and what is not. For ecample, imagine you have two nodes: One for “Red Spots” and one for “Measles”. Some might say that when you observe red spots on a child it is very likely that the child has measles, so you should add a link from “Red Spots” to “Measles”. This, however, is terribly WRONG! The relation between red spots and measles is that measles causes red spots. The link should go from “Measles” to “Red Spots” (because it is a causal link).
Always make sure that the causal links you add are causal.