After the file is opened, the system needs to know where it should start processing the file. You can specify the initial file position with the POSITION parameter.
The default is to start just before the first record in the file (the first sequential read operation reads the first record). But, you can tell the system to start at the end of the file, or at a certain record in the middle of the file using the Override with Database File (OVRDBF) command. You can also dynamically set a position for the file in your program.