The aux mix (stereo?) used for streaming should be set to "postfade". All faders should be set to unity, so that the aux mix reflects 1:1 what is done in the Main mix. This allows you to change volumes of individual channels as needed ("more lead vocals for the live stream"). If you are mixing stereo, do not forget to pan stereo channels manually. By default they are set to mono.
Pre1 and Pre2 are the other options for aux and matrix mixes, which means pre-processing or post-processing (EQ, compressor etc. included).