System Configuration : I/O Setup : I/O Setup Options : Default Output Bus

Default Output Bus
You can specify the default output bus path assignment for new tracks, in each available format.
The Default Output Bus selector is available in the Output and Bus pages of the I/O Setup.
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The Default Output Bus can be set for internal mix bus paths, as well as for output bus paths.
To specify a default output for new tracks in the I/O Setup:
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Click the Default Output Bus pop-up menu and select a format and output bus path.