When you configure a FileName port, the Integration Service overrides the Output Filename session attribute with the value in the FileName column. If the FileName column value is NULL for a row, the row is an error and the Integration Service does not process it. If the FileName column is NULL after a transaction boundary, the Integration Service names the output file with the default output file name.
The FileName column must contain a unique value for each transaction. If the FileName column value does not change between transactions, the Integration Service overwrites the flat file target.
If you do not connect the FileName port in the mapping, the Integration Service generates one target file and uses the output file name configured in the session.