App Review Board
The App Review Board is Apple's appeal route for a rejection you believe applied a guideline incorrectly, and it is a different channel from the reviewer thread in Resolution Center.
Use it when the disagreement is about interpretation. The reviewer says your app duplicates existing functionality and you can show it does not. They cite a subscription rule that does not cover your model. They read a feature as doing something it demonstrably does not do. Those are arguments about the rule, and the Board is who decides them.
Do not use it when the rejection is correct and you would rather it were not. If the app really does what they said, an appeal spends time you could have spent fixing it. The Board is not a second chance at the same conversation with friendlier reviewers.
You submit through Apple's developer contact form, in the App Review section, choosing the App Review Board option.
Write it as a case, not a complaint. Name the guideline, quote the exact rejection text, state precisely what the app does, and give evidence: a screen recording, a specific screen, working credentials. Keep the tone flat. The people reading it did not reject you.
Apple publishes no turnaround and no outcome statistics, and appeals are slower than resubmitting. If a fix exists, ship the fix and appeal in parallel.
See also: Resolution Center, App Review, expedited review.