specify that a fact is always a consequence of another;
defeasible rules
specify that a fact is typically a consequence of another;
undercutting defeaters
specify exceptions to defeasible rules.
A priority ordering over the defeasible rules and the defeaters can be given. During the process of deduction, the strict rules are always applied, while a defeasible rule can be applied only if no defeater of a higher priority specifies that it should not.
D. Nute (1994). Defeasible logic. InHandbook of logic in artificial intelligence and logic programming, volume 3: Nonmonotonic reasoning and uncertain reasoning, pages 353–395. Oxford University Press.
G. Antoniou, D. Billington, G. Governatori, and M. Maher (2001). Representation results for defeasible logic.ACM Transactions on Computational Logic, 2(2):255–287.
