I don't really understand the point of some of those functions either. I assume they're for the in-deck editing abilities that I've never bothered using. I adopted non-linear editing years ago, never needed analog deck editing. (Not from these decks, at least -- my analog editing was on other equipment some 20 year ago.)
The "MIX" setting will mix your linear and HiFi channels, and it sounds like audio played through a tunnel. It's awful. Echos can be present, too. Don't use it.
HiFi L and HiFi R are left and right channels only. I almost never have need for these, except in cases where one channel of the VHS tape audio is damaged and the other is not. (And in those cases, I split the RCA audio output with a wire splitter, to be sure the digital capture has the remaining audio available in both L and R channels.)
HiFi should be used unless there is damage to the trak, such as buzz and fluttering noises in the audio.
Norm / Mono / Linear -- the linear audio track, usually mono only -- is the "backup" on HiFi tapes. Or the only track on others, especially on home VHS camera recordings. The linear track won't get the buzz and fluttering like HiFi will, unless the tape is very damaged physically. An "eaten" tape, for example. HiFi is recorded on tape using a hack anyway, and that's why it is more touchy.
The downside to linear audio is it sounds muted and flat. Only use it when necessary, or when it's the only thing available.
Some of this is already explained on the VCR / TBC guide: http://www.digitalfaq.com/guides/vid...k-hardware.htm