Русско-англо-русский юридический словарь

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)

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)

defective performance. Complete or incomplete (partial) performance that does not fully satisfy the requirements of a contract. (BusinessDictionary)

Non-performance is failure by a party to perform any of its obligations under the contract, including defective performance or late performance. (Article 7.1.1 of UNIDROIT PRINCIPLES 2010)