Field help (BSIREFBAN) 

Enter a unique code or string appearing in the bank statement file to identify this bank account.

This field is optionally used in the context of the advanced bank statement import process. Depending on the setup, the bank statement import uses this information to automatically find the Sage X3 bank account belonging to the bank statement file. Typically, this is needed when importing statement files containing multiple statements from different bank accounts for the same bank in one file.

Refer to the Advanced Bank Statement how to guide for more details.