A more thorough explanation:

Definition: The integration rule is a legal principle that states that if the parties involved in a contract have put their agreement in a final document, any other action or statement is irrelevant and cannot be used to determine the terms of the contract. This rule is also known as the parol evidence rule.

Example: Let's say that John and Jane enter into a written contract to sell a car. The contract specifies the make and model of the car, the price, and the date of delivery. Later, John tells Jane that he will throw in a set of new tires for free. However, according to the integration rule, this statement is immaterial and cannot be used to change the terms of the contract. The only terms that matter are those that are included in the written agreement.

Another example: Suppose that a company hires an employee and provides them with an employment contract that outlines their salary, benefits, and job duties. Later, the employee claims that the company promised them a promotion after six months. However, if this promise is not included in the written contract, it is not enforceable under the integration rule.

The integration rule is important because it helps to ensure that contracts are clear and unambiguous. It prevents parties from using outside evidence to change the terms of a contract after it has been signed. This rule provides certainty and predictability in business transactions, which is essential for a stable and functioning economy.