The way this works is that you supply the name of the “I/O handler” program that handles the file operations in a keyword on the F spec (PGMNAME). A standard parameter list is sent to that program whenever any I/O operation is performed on the file.
The standard parameter list includes the action or operation that was requested (e.g., open, read, write, update, etc.), a character field representing the record data for the file, a status code that you can use to signal conditions (e.g., an error or end of file) and an error code so the I/O handler program can signal specific errors back to the “calling” program.
If the I/O handler program needs additional information not included in those standard parameters, additional parameters may be passed. These are specified with another F spec keyword (PLIST).