To put it simply: (1) If the parties are considering a full integration of the terms of the contract, no Parol evidence is allowed under the agreement. (2) If the parties are considering a partially integrated agreement, it is not permissible to obtain evidence from Parol that opposes something integrated. And (3) if the Parol evidence is a guarantee, that is, it relates to another agreement and is not in contradiction with the integrated terms and are not terms that a reasonable person would always naturally incorporate, then the rule does not apply and the evidence is admissible. Although its name indicates that it is a rule of procedural evidence, courts and commentators agree that the Parol evidento rule is a material right of contracts. The second agreement was outside the evidence, but a court authorized its introduction for two reasons. First, the oral agreement did not contradict the written and fully integrated option agreement. Second, an agreement with a commission is not something that parties in the same position would normally include in a real estate purchase agreement. The Parol rule of evidence can therefore be simplified as an “external rule of evidence.” External evidence cannot be used in the event of a written contract. Like most law courses, this one has many qualifications and exceptions. The third and final rule of admissibility is that under UCC 2-202, parol evidence cannot be contradicted by a writing conceived as a “definitive expression” of the integrated agreement, but by (a) a commercial/commercial/performance history and (b) evidence of consistent complementary conditions, unless the letter is a full and exclusive statement of contractual terms.
Sometimes a term is ambiguous and needs to be clarified from the outside. What is “wood,” for example? When developing a treaty, the parties sometimes forget to define such a key concept. In type v. Smith, the parties have denied the importance of this clause in an agreement.  The Mississippi Supreme Court allowed the plaintiff to introduce parol evidence to show the importance that the parties themselves placed on the words of their own written contract.