To create a data dictionary, you specify the "-Fx" or "-Fxa" option at compile time. When you specify the "-Fx" option, an ".xfd" file is created for every indexed file contained in the compiled program. When you specify the "-Fxa" option (ACUCOBOL-GT Version 4.1 or later), an ".xfd" file is created for every indexed and relative file contained in the compiled program.
Once you compile with one of these options, each COBOL file has a corresponding ".xfd" file. The "-Fx" and "-Fxa" options create ".xfd" files without changing anything in the object code; thus XFDs are fully portable, meaning no recompilation is necessary if you change data sources or hardware.
Creating ".xfd" files at compile time offers two significant advantages:
The XFDs contain information about the structure of the COBOL files, the names of the fields, their format, and so on. However, this information is only a subset of the information available in most Windows applications. To map your COBOL fields to meaningful and useful application tables, you must add additional information to your data dictionary. To add information to your XFDs, or to change the default names of the existing fields, you must use directives, or optional comments, in the FD section of your COBOL data file. These directives are explained in section 6.3, "Using Directives".
More:
6.2.1 How the Data Dictionary Is Formed
6.2.2 Defaults Used in XFD Files
6.2.3 Examples of Default Mapping