Русско-англо-русский юридический словарь Игоря Буяльского

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).

A default rule is one that governs unless the parties contract out of it. In contrast, a mandatory rule is one that governs despite a contract term to the contrary, that is, a rule that cannot be avoided by contract. One can identify which laws are default and which are mandatory by examining the sorts of contract terms that are, and are not, enforceable. For example, the legal rule that the place for delivery in a sale of goods is the seller's place of business is a default rule because parties can make an enforceable contract requiring delivery at some other location. In contrast, the legal rule giving a consumer the right that goods purchased not be "in a defective condition unreasonably dangerous to the user" is mandatory because it applies no matter what the contract terms say. The distinction between mandatory and default rules is fundamentally important because it reveals the extent of contractual freedom. Mandatory rules limit the freedom of contract, while default rules permit it. (Arbitration Law in America: A Critical Assessment edited by Edward Brunet).

In legal theory, a default rule is a rule of law that can be overridden by a contract, trust, will, or other legally effective agreement. Contract law, for example, can be divided into two kinds of rules: default rules and mandatory rules. Whereas the default rules can be modified by agreement of the parties, mandatory rules will be enforced, even if the parties to a contract attempt to override or modify them. (Wikipedia).