Any appeal under any law is an application to a higher court for a reversal of the decision of a lower court. Appeals arise when there are any legal disputes.
Tax laws (or any law) impose obligations. Such obligations are broad of two kinds: tax-related and procedure-related. The taxpayer’s compliance with these obligations is verified by the tax officer (through an audit, anti-evasion, examining, etc.). Sometimes there are situations of actual or perceived non-compliance. If the difference in views persists, it results in a dispute, which is then required to be resolved.
The initial resolution of this dispute is done by a departmental officer by a quasi-judicial process resulting into the issue of an initial order known by various names -assessment order, adjudication order, order-in-original, etc.
GST Act defines the phrase “adjudicating authority” as any authority competent to pass any order or decision under this Act, but does not include the Board, the First Appellate Authority and the Appellate Tribunal. Thus, in a way, any decision or order passed under the Act is an act of “adjudication”.