Each data item in your COBOL FD must correspond to a field in the database. You may have columns in a table for which no data item exists in the COBOL FD, but not vice versa. You may not have data items in your FD with no counterpart in the database.
This correspondence happens automatically when the database is newly generated by the Acu4GL or AcuXML interface. If the database is pre-existing, you should check for the correspondence, and use directives if necessary to make it complete.
Data dictionaries may be built directly from your source code with no directives if the default mapping rules described earlier are sufficient for your situation. If you would like to override the default mapping behavior, or map a field to a different name, then you would add directives to your COBOL code.