An Appraiser is a person who calculates the value of something like a car, house, jewelry, etc…There are speciality appraiser that will have extensive credentials, experience in a certain field of appraising certain types of items.
However, when it comes to an appraisal clause in an insurance policy and appraiser is usually required to be a disinterested party that both the insurance company names and the insured names and they work together to see if they can reach a settlement on scope related issues. Coverage decisions are usually left to the courts and are usually litigated. If the two appraiser can not reach an agreement then an agreed upon Umpire is selected to inspect and appraise the home. Once this happens if 2 out of 3 of the appraisal panel come to and agreement then the insurance settlement is considered binding and an insurance settlement payment is issued. (This may differ by policy and by state.)
Experienced Public Adjusters has expert appraisers if needed if appraisal in invoked by either party.