batsbrew
still don't know what the h8ll a mix bus is.....
At the risk of taking a CH thread too literally/seriously... I've always taken "mix bus" to mean a bus that is taking the entire mix into it. That's where you'd apply effects to polish everything up with light compression/EQ/limiters/etc...
I usually set up a "Pre-Master" bus just before the master for this purpose and leave the master bus clean... but I'm no genie ass.