I haven't played in a few years but this is how I remember the system working:
RDO matches people into separate lobbies based on what version of the game they're running and this check is done by hashing one or more files in the install directory. By adding junk data to one of those files you more or less guarantee that you'll only ever encounter other people who have the same junk data added.
It's basically the dark souls password system with extra steps.
I haven't played in a few years but this is how I remember the system working: RDO matches people into separate lobbies based on what version of the game they're running and this check is done by hashing one or more files in the install directory. By adding junk data to one of those files you more or less guarantee that you'll only ever encounter other people who have the same junk data added. It's basically the dark souls password system with extra steps.