“Muxing” means combining different things (like video stream, audio stream, or subtitle) into one single media container format like MKV, AVI, etc. media container you have.
This site can help you.
RE:
what are mixed subtitles?
Do you mean “internally”? that’s to say. after mixing, the external subtitles are encoded or become inherently part or internal of the media container.
“Muxed” = closed captions (and almost always unremovable).