The parol evidence rule is a law of evidence which is designed to support the validity of an obvious written contract over any other evidence, assuming that the written contract does not appear to have been tampered with or made falsely, and does appear to be complete and in effect.
The parol evidence rule would thus concern situations in which an individual claims that an oral conversation took place which provided terms for the agreement in question which might have been different from the terms proposed on the actual contract. In such a case, the parol evidence rule would act as a law of evidence and would disallow the evidence of the oral conversation, as the written document would override the written conversation.
The parol evidence rule makes several important distinctions which are important to understand in order to fully realize how it is applied as a law of evidence. The parol evidence rule, for example, sets out that the contract under consideration must be absolutely final and complete for the parol evidence rule to apply in full.
If the contract is partial, and is not complete, then this law of evidence would apply in terms of disallowing evidence outside of the contract which might contradict the written document, but it would not act as a law of evidence in preventing the introduction of evidence which added to the contract. A full contract, under the parol evidence rule, inherently disallows the introduction of any evidence which would alter the contract in any way.