if I remember right the ALT output mirrors either the BUS 1-8 outs or AUX 1-8 Outs.
Doesn't "mirror" (suggesting fixed). It's assignable via Setup>Network: Alt I/O (outputs) 1-8:
In the middle of the menu, a set of eight pulldown menu buttons will route audio signals from Aux 1–12, Bus 1–8, or L/R output through any of the eight outputs available on an I/O card installed in the Alt I/O slot.
There are additional setting controls for Sample Rate conversion and Status Bits in the Alt 1-8 section in the lower left of the Setup>Digital I/O menu.