after opening the mixdown song dialog box, there is an option “master channel” and another right under it “substitute tracks…” Have read the help and still don’t understand :
1. what is master ch
2. do you check or uncheck it if you’re going to substitute tracks. the help really isn’t clear on this.
thanks
fivestring
The master channel is what happens to everything after it’s all mixed (“summed”) together. You can put effects on the master channel, and adjust the master channel fader. This checkbox affects whether these are used or not. When replacing tracks, it’s usually a good idea to uncheck this box, because you only want the stuff you did to the tracks you’re replacing, and not the stuff you’re doing to the whole mix.
To see the master channel in V4, look near the bottom of the timeline and check out those little buttons until you find the one that says “Master”, and click that. The master channel stripe will appear (where the track mixer stripe appears when you click on a track). There’s the master fader and master effects box. You can also see the master channel in the “Mixer” view.
However, I suggest not to use the “substitute tracks” feature. I find that when I’m re-rendering a track or set of tracks for some reason, it’s best to leave them there and just mute them, and import the new track. I can always delete them later. But if you made a mistake when mixing down and they get deleted, and then you do something you don’t want undone, you can’t undelete those old tracks.
Note that you can also “minimize” a track by clicking in the upper left corner near the track name.
many thanks learjeff— for the input on master channel and substitute tracks. it was quite helpful; you should’ve written the “help” for nTracks. 
I wonder sometimes if Jeff don’t know how to use N better than Flavio does …
After all, you don’t see the guy who designed and built mr Schumacher’s ferrari standing on the podium all the time …