The FILEOPT parameter is used with XCOM to determine whether the destination file is to be created, replaced, or appended to. The CREATE and REPLACE values for this parameter will obviously create or replace the destination file regardless of which operating system that the destination file is on.
What is not so obvious is the correct value to use to append to an existing file. Most operating systems expect a value of APPEND for this (as you would expect) but some operating systems (eg. MVS) expect a value of ADD instead.
This article written by Stephen Chapman, Felgall Pty Ltd.