TRN or Temporary Reference Number is a unique 15-digit reference number that is generated when you successfully submit all the fields of PART A (first page) of the new registration application and successfully
validate your mobile number and e-mail ID by correctly entering the respective OTPs.
Your TRN is sent to you via SMS and e-mail. It is valid for 15 days from the date of creation.
After TRN is generated, note it down and access PART B of your new registration application on the GST Portal in the pre-login mode by entering the TRN.