It's about AL-1 Exi, 4–3c: Sub OSC/Audio Input
This list is a wrong one. Because Audio Input 3 and 4 do not exist ! The right list is :External Audio [Audio Inputs 1…4,
S/P DIF L, S/P DIF R]

So, we notice that there is two USB inputs. But the manual does not explain what they are and how to use them (what kind of file, etc....)