Field help (COACREAUT) 

If the box Automatic creation is ticked and if the account to use via the accounting interface or the import does not exist, it is created automatically by the processing and takes the previous account as a template.
The account thus created can be identified by its title which contains the number of the source entry surrounded by three X.

For instance:
XXX FC FAC-ASN0101-005 XXX

It is recommended to return to the record of the account to validate or enter the default values given by the processing: VAT management, analytical tracking...