In case of new registration, TRN will be allotted when we provide basic details like PAN,email,mob no etc.
ARN will be issued when the application is filled and submitted on the portal.Once an application is submitted ARN can be used for future reference. TRN can be used for making B2B purchases till the application is approved and GSTN is allotted by the Department.When GSTN is allotted,the invoices issued with TRN should be updated with GSTN before the supplier file his GSTR1.