Presumption of correctness
The legal starting point in an appeal: your assessment is treated as correct until proven wrong. The person appealing, usually the owner, carries the burden of proof.
New Jersey example
Telling the county board that your taxes feel too high will not lower your assessment. You need evidence of what your home was worth on the assessment date, such as sales of similar homes. If your evidence does not overcome the presumption, the assessment stays as it is.
Official source
Handbook for New Jersey Assessors (NJ Division of Taxation, revised July 2024)
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.