OVRDBF is used to redirect input/output
The RPG or COBOL program uses FILEA but you need it to
– use FILEB (which is structurally the same)
– use LIBB / FILEA
– use the results of an opnqryf as if it were FILEA (be sure to include SHARE (*yes)
– use a specific member in a multi-member file
The Override with Database File (OVRDBF) command is used to (1) override (replace) the file named in the program, (2) override certain parameters of a file that are used by the program, or (3) override the file named in the program and override certain parameters of the file being processed. Parameters overridden by this command are specified in the file description, in the program, or in other previously issued file override commands. This command applies to physical files, logical files, and distributed data management (DDM) files.
To override (replace) a file named in the program, specify the name of that file in the FILE parameter, and specify the name of the file that overrides it (the file to be processed by the program) in the TOFILE parameter. The other parameters of this command can be used to override parameter values contained in the file description of the overriding file.
To override only certain parameters of the file named in the program, instead of replacing the entire file, specify the name of the file in the FILE parameter and specify the *FILE value for the TOFILE parameter. Then use the other parameters of this command to override specific parameters of the file. Parameters that are not specified do not affect parameters specified in the file description, in the program, or in other previously issued file override commands.
You can have more information on this on following link for OVRDBF.