What a failure means
Failures are the sharpest of the three types: patterns W3C has documented as definitively NOT meeting a rule. If your page contains failure F65 (an informative image with no alt attribute), then rule 1.1.1 is not met. No argument, no interpretation.
The failure catalog is effectively WCAG's case law: 90 documented ways sites actually break the rules, distilled from decades of real-world patterns.
Why failures carry the most weight
Audits and expert reports cite failures because they are the closest thing WCAG has to a proven violation. "Your site exhibits F65 on these six pages" is a claim that can be verified by anyone, which makes it powerful evidence for whoever's side it supports.
That cuts both ways: clearing every documented failure, with evidence, is equally verifiable, and it is exactly what good fix evidence documents.
In audits
Our findings map to documented failures wherever one applies: each finding cites the failure it matches and links the sufficient technique that resolves it. That pairing is what makes a report able to hold up under scrutiny and still tell a developer exactly what to do.
If you read only one technique type, read the failures: they are the shortest honest list of what actually gets sites sued.