A: ALT I/O stands for Alternate input and output. This slot can output any combination of auxiliary sends 1-8, sub busses 1-8, or the master L-R mix bus. Inputs to this slot will appear on RET 1-8 channels on the Effects fader bank, which are without the standard channel dynamics found on channels 1-48. This slot is the recommended I/O platform for bussing signals between two (or more) cascaded D8B's. Typical uses of this slot would be for digital auxiliary sends and returns, sending a surround mix to a multi-track recorder, bussing pairs of stereo sends to different devices, or receiving auxiliaries or busses from a slave D8B.
You can use any of the approved Mackie Digital Systems Tape I/O cards installed in this slot.