A certificate number is a 10 digit alphanumeric value. It is only mandatory if the Reason for Lower/ No deduction of TDS is mentioned. This certificate is issued by the Assessing Officer of the Income Tax Department to payee/ deductee. This certificate is issued to payee each year. 

It is important to ask for a certificate from payee/ deductee before deducting TDS at a lower rate. Since payer/ deductor will be penalized if false or invalid certificate number is provided.