Form 15CB is the certificate by Chartered Accountant.
Section 195 of the Income Tax Act, 1961 mandates deduction of income tax from payments made or credit given to non-residents at the rates in force. Further as per the Reserve Bank of India mandate, except in the case of certain personal remittances, any remittance to a non-resident requires the remitter to furnish: 1) an undertaking along with 2) a certificate from an accountant.
The purpose of the undertaking and the certificate is to collect taxes at the stage when the remittance is made as it may not be possible to recover the tax at a later stage from non-residents.