Each field in the database must correspond to a data item in your COBOL FD. You may not have any database table fields that have no counterpart in the COBOL FD. The reverse is not true; there may be COBOL fields which the XFD maps to the database table columns, but do not appear as column names.
This correspondence happens automatically when the database is newly generated by the Acu4GL 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.