ACUCOBOL-GT data dictionaries are based on your COBOL FDs. For this reason we call them extended FDs or XFDs. Each dictionary describes all of the fields for one file.
Directives are optional comments that you can use in your FDs to control how things look on the database side. Many applications won't use directives at all. They're most commonly used when a site intends to do a lot of work with the database management system outside of the COBOL application, and wants to control how the database table is built.
Directives are special comments placed into an FD in your COBOL source code. They guide the building of the data dictionaries, which in turn guide the building of the database table.
Each directive includes the letters XFD. These three letters indicate to the compiler that the comment is to be used in Data Dictionary generation.
Directives offer you a great deal of control over how the database table is built. Among other things, they enable you to:
Directives are always placed within a COBOL FD. They do not affect Procedure Division I/O statements, and they do not change your COBOL fields in any way. Rather, they guide the building of the data dictionaries, giving you a measure of control over the way COBOL data is mapped to database fields.
Data dictionaries may be built directly from your source code with no directives if the default mapping rules described in Chapter 3, "Data Dictionaries" and Chapter 7, "New and Existing Databases" 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.