I will explain my question with an example: Let's say I have 10 tracks. I want the 10 tracks to be my main mix (the one I listen with my headphones). But I would also want to take 4 of the 10 tracks, mix them together and output them to a stereo output on line 3 and line 4 of my soundcard. This "sub-mix" will go to other hardware that doesn't need the whole mix.
I want the 10 tracks to stay in the "global" mix that we hear when we listen, and I want 4 tracks to mix and output to a studio output.