Compulsory registration for commission agents arises only when the principal is taxable and the agent's supplies are taxable. A commission agent is mandatorily required to register under the clause for persons making taxable supplies on behalf of others only when the principal is a taxable person and the supplies made by the agent are taxable; an agriculturist supplying produce from cultivation is not a taxable person, so agents acting for such agriculturists are not compulsorily registrable under that clause. Separately, a commission agent liable to pay tax under the reverse charge mechanism must obtain compulsory registration under the provision for reverse charge payers.