GST under the Constitution is very widely defined. A definition of GST has been specifically added in the Constitution, which is Clause 12-A in Article 366. It defines GST to mean any tax on supply of goods or services. There is only one exclusion from this definition namely, alcoholic liquor for human consumption. Other than alcoholic liquor, anything else can be covered within scope of GST if it is supplied. The answer for this or rather the reason for this is another factor which is again on account of an amendment. 

There is a definition of services which has been enacted in the Constitution. Clause 26-A has been inserted in Article 366 which defines services and the definition is, that services means anything other than goods. 

Therefore, either a commodity is taxed under GST as goods or if it is not matching the description of goods, it will be taxed as a service owing to this definition. Therefore, the definition and concept of GST under Constitution is very wide.