Rule 31 of GST Rule 2017.Residual method for determination of value of supply of goods or services or both –
Where the value of supply of goods or services or both cannot be determined under rules 27 to 30, the same shall be determined using reasonable means consistent with the principles and the general provisions of section 15 and the provisions of this Chapter:
Provided that in the case of supply of services, the supplier may opt for this rule, ignoring rule 30.
One example of the residual method is valuation on the basis of per unit when the cost of manufacturing cannot be determined in principle. The number of man hours required to complete a job can be another example of such valuation method.