When oral admissions as to contents of documents are relevant

Oral admissions about a document's contents are irrelevant unless the party proving them shows they are entitled to give secondary evidence of that document, or unless the document's genuineness is in question. So you cannot rely on someone's spoken account of a paper instead of the paper itself, except where law permits secondary evidence or authenticity is disputed.

Priya sues a shopkeeper saying a written receipt promised a refund, but she has lost the original. She tells the court orally what the receipt said. Her oral account of the receipt's contents will be irrelevant unless she can show she is entitled to give secondary evidence (for example, the original is lost) or unless the shopkeeper disputes the receipt's genuineness.