Forensic only means it is applicable to court or law proceedings. In that sense, a forensic appraiser would include most condemnation appraising. An appraiser defending their appraisal before a court of law in a legal proceeding, such as a divorce is likewise acting as a forensic appraiser. Otherwise, a more narrow definition might be someone who acts in a legal review capacity. I would think this normally combines the person's expertise in dissecting the opposition's appraisal or providing support for a favorable appraisal.
In one case I know where an appraiser submitted an appraisal for a taxpayer contesting the valuation of a golf course by the assessor. The assessor did not second guess themselves by hiring an appraiser to appraise what they had done, only hired a golf course expert appraiser to testify that the proper method for appraising a golf course is the COST approach. The taxpayer had relied upon an appraisal by a person who had no business taking on such an assignment and who weighted the income approach.
The expert sucessfully impeached the testimony of the appraiser and the courts ruled in favor of the assessor. He acted more in the capacity of a consultant rather than an appraiser, pointing out the weaknesses of the appraisal by the taxpayer and suggesting a method to get that point across. He did not draw his own value conclusion.
Often if the value conclusion is in serious dispute, I would think a forensic appraiser should be able to review the other appraiser's work within the scope of RE consulting rather than as Review Appraiser, but one might need to qualify an assignment within the scope of Std. 1,2 or 3,4.
I don't recall specific courses related to forensic appraising outside the Court testimony and condemnation courses.