Tvheadend

Full Mux RX mode grabs the whole mux signal instead of demuxing it to get a specific channel. Last I heard, it wasn't useful for most people unless you were deliberately trying to get the whole mux (e.g. for debugging purposes)... once upon a time there was a suggestion of using it to speed up channel changing on the same mux (because all channels would be being received at the same time), but I don't believe that was implemented.

Keep FE open, I'm less sure about. I believe it locks the adapter front end, but I don't know why... I've guessed in the past that it's to prevent the adapter going to sleep, or stop any other application from querying it.

I don't believe you would normally need either option enabled, though.