specify that a fact is always a consequence of another;
specify that a fact is typically a consequence of another;
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. In Handbook 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.