Apple Soundtrack Pro 3 - Manual (Page 127)

The Master Bus
The Master bus represents the final mix from Soundtrack Pro: what will be heard from
physical output jacks. On a signal routing basis, the Master bus is practically a mirror
image of the connected hardware output device. It has a single volume control but does
not have panners. You can apply effects to the Master bus, but you cannot automate
those effects. Effects applied to the Master bus are placed across all channels.
For more information about the Master bus, see Using the Master Bus .
127 Chapter 5 Working with Multitrack Projects