Viva voce evidence is oral testimony — live evidence given by a witness under oath, in the witness box, subject to cross-examination. In a Tax Court of Canada appeal it complements the documentary record, supplying what the documents leave out: what a transaction was for, why a decision was made, and what happened where no record was kept.
A judge can accept sworn testimony as direct evidence of what occurred, and where the Crown has no real evidence to contradict credible oral testimony, an assumption resting on suspicion or inference may not survive. Because so much of an appeal turns on viva voce evidence explaining the documents, credibility is frequently decisive.