To OP: I posed a similar question a little while ago. See here: https://hydrogenaud.io/index.php/topic,110670.0.html
It seems the terms 'back' and 'side' may be arbitrary terms for the same placement position.
Exactly. I don't get the problem. Whether a codec refers to a channel as side or rear in a 5.1 setup, they go to the same speaker.
So is this just a semantics complaint, or is there an actual problem? I understand that ffmpeg has some issues encoding Opus 5.1, but that's ffmpeg's problem. Are there are other problems, perhaps upmixing to 7.1 setups?