If the file is program-described, the file
does not need to exist. 
If the file name in the RPG program is longer
than 10 characters,
you must specify the EXTFILE keyword.
If the file is externally-described, you must specify
the EXTDESC keyword. 
Otherwise, RPG will open the file whose name is the same
as the name of the file in the RPG program. This file
(or an overridden file) must exist when the file is opened. 
When files that are not defined by the USROPN keyword are opened at run time, they are opened in the reverse order to that specified in the file description specifications. The RPG IV device name defines the operations that can be processed on the associated file.