PreSonus FireBox - Manual (Page 23)

4 MIXER
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Input (software playback channel) – This is the audio return
from your audio software. The same functions are available as
the other inputs including PAN (balance between left and right
channels), LEVEL, SOLO and MUTE. This is a stereo channel.
This channel picks up the audio stream that is sent from your
software and enables you to mix it with the direct input of your
FIREBOX.
PLAYBACK – Indicates the stereo stream output channels from
your software.