Set A_DB2_DATASOURCE to the exact name of the host data source. You can set this variable in your COBOL ACUFH configuration file if using only one data source.
If you don't know the data source name in advance, or if you intend to use more than one data source, you may set the data source name dynamically at run time. In your COBOL program, you would add code similar to this prior to the statement that opens the file:
SET ENVIRONMENT "A_DB2_DATASOURCE" TO "data source name"
There is no default value for this variable.