The significance of the fields
The Bank Statement Mapping window is useful when you wish to have certain fields in the Import Bank Statement window be automatically filled in based on the fields present in the import. For example, if, in the description of the transaction, you encounter the word "commission", then the corresponding charge will automatically be selected.
Mapping Configuration
If you create multiple mapping rules for the same source field (and same source field value), with different values for the destination field, then only the record created last will be taken into account.