43 Pa. Stat. § 829
Any decision made by the department or any referee or the board shall not be subject to collateral attack as to any application claim or claims covered thereby or otherwise be disturbed, unless appealed from.
Subject to appeal proceedings and judicial review, any right, fact or matter in issue which was directly passed upon or necessarily involved in any decision of a referee or the board or the Court and which has become final shall be conclusive for all purposes of this act and shall not be subject to collateral attack as among all affected parties who had notice of such decision: Provided, however, That whenever an appeal involves a question as to whether services were performed by a claimant in employment or for an employer or whether remuneration paid constituted wages, a decision thereon shall not be conclusive as to an employing entity's liability for contributions unless the employing entity was given special notice of such issue and of the pendency of the appeal and was afforded a reasonable opportunity by the referee or the board to adduce evidence bearing on such question. No finding of fact or law, judgment, conclusion or final order made with respect to a claim for unemployment compensation under this act may be deemed to be conclusive or binding in any separate or subsequent action or proceeding in another forum.
43 P.S. § 829