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In contract law, we can sort the rules into two sorts, "default rules" and "mandatory rules."
Here's an example. The Uniform Commercial Code (or UCC, the codified law of contract that applies to contracts between businesses as a matter of state law in the United States) creates a duty to act in good faith--this is a mandatory rule, because this duty cannot be disclaimed by a contractual provision. The UCC also includes an implied "warranty of merchantability," that attaches to contracts, but can be waived by agreement--this is a default rule.
Grasping this distinction is important for at least two reasons. First, unless you know whether a given rule of contract law is a default rule or a mandatory rule, you don't really know the law. And it isn't always clear whether a given rule is one or the other: the usual tipoff is language like, "unless the contract provides otherwise" or "absent an agreement to the contrary."
(Legal Theory Lexicon 050: Default Rules and Completeness).