Apple Logic Pro 9 - Manual (Page 779)

An aux channel strip appears to the right of the instrument channel strip, already assigned
to the inserted multi-output instrument.
4 Repeatedly click the Add button (+) to create more aux channel strips, for all stereo or
mono outputs available to the instrument plug-in.
You should create only as many aux channel strips as required for the number of outputs
used by the multi-output instrument.
Following the creation of the first aux channel strip for your multi-output instrument, a
Delete button (–) appears beside the Add button (+).
Click the Delete button (–) to remove aux channel strips.
Using Output Channel Strips in the Mixer
You use output channel strips to assign submixes to physical hardware outputs of your
interface. You do this by routing multiple channel strips to a common output path via
their Output slots.
779 Chapter 27 Mixing