Substantial question of law enables appellate review and confines the hearing to the formulated legal question. Substantial question of law is the threshold for appeals from the Appellate Tribunal to the High Court; certain categories of orders relating to interstate treatment and place of supply are excluded. Appeals must comply with filing time limits, prescribed fee, and a memorandum stating the substantial question, though the High Court may extend time for sufficient cause. The Court formulates and decides the question, limits the hearing to that question while retaining discretion to consider other substantial questions, may determine issues left undetermined or wrongly decided by the Tribunal, and requires a bench of at least two judges.