Subgroups. The four blue buttons next to the fade slider in each mixer unit allow you to link the unit to one of four Subgroups. For example, all Mixer units with button 1 pressed will be controlled by Subgroup 1.

Open the Subgroups window from the windows menu. This allows several tracks to be locked together, so that moving the slider in the Subgroups window fades all of them at once.

The Subgroup controls the volume, mute and solo settings of all the units that are linked to it. The Mixer volume setting is multiplied by the Subgroup setting to give the result. For example, if the fader in Track 3 of the Mixer is set to 50%, and Track 3 is assigned to a Subgroup which also has a fader set to 50%, then the output will be 50% of 50%, which is 25% of full volume. Or, 0.5 * 0.5 = 0.25.

The same applies to the Master Volume. If this was also set to 50%, the final output from that track would be 50% of 25% which is 12.5% of full volume.