Tracks are first read through and the original sound created.
This sound then runs through the 4 FX slots on the track itself.
In the mixer, if it is an audio track, there are AUX sends that allow you to send a given amount of that signal to the FX in any of the AUX slots (after going through the Track FX).

The subgroups are assigned at the Port level; right click a track and select one of the SubGroups as an output. (see image below)
You can send as many tracks as you like to any Subgroup.
The Subgroups are then summed to the Audio Output selected.
Hope that helps ..

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