After you have completed the mapping look below the Data Mapped to QuickBooks Online grid and select the options button. Select the option for your selected import type. Please note that there is a known bug in the Intuit App Center as it relates to auto assigning numbers on journal entries. We have reported this issue to Intuit and are awaiting a fix.
If you use this option you will still need to provide a value for the QuickBooks field, RefNumber, on each line. Transaction Pro Importer for QuickBooks Online (TPI) requires this as this is how TPI determines how to group multiple lines onto the same transaction. If you select any of the options highlighted above this option is only used by TPI for internal grouping purposes.
You can map the QuickBooks field RefNumber to any field so for instance if a new transaction is to be generated when a new customer appears in your import file then just map the import file column that contains the customer name in it to both the customer and refnumber fields.