IBM System i AS400 Field Description Files
A description file is a text file in ASCII format that contains descriptions of each field in a corresponding data file, which allows you to keep data in the intended format when sending data to the host. Field description files are created by default during data transfers to your PC.
The description file has the same name as the data file, except for the extension, which is
.FDF. You'll see its name in the Local file names box in the Transfer dialog box.
If the file you're sending did not originate on the host, and has never been transferred to the host, you must create a description file before you can send a PC file to the host.
Your system administrator may have created a generic description file that resides on the host and can be downloaded, modified, and saved with a new name. (Check with your system administrator to see if this option is available.) Or, you can create your own description file. Refer to your IBM System i (AS/400) documentation for more information.