The IRS appeals process is not terribly mysterious. The proceedings are informal. The taxpayer or representative meets with the Appeals Officer, they sit across the table from one another and discuss the facts, the proof of those facts, the law and whether the law supports the taxpayer or the IRS.
The facts of a case are fixed, how those facts are proven, however, will vary from case to case. Perhaps the taxpayer had the foresight to keep detailed books and records. Perhaps more unconventional documents will be needed to establish certain facts. If the taxpayer has witnesses that would testify on their behalf, those witnesses could provide affidavits attesting to the truth of the taxpayer’s position.
Based on these aspects of a case, the taxpayer and appeals officer will likely find some basis on which to settle the case. Any such settlement will be agreed to and documented by the IRS and taxpayer. Following that documentation, both sides will be able to rely on the settlement in any situation related to the years and items at issue in the case.