JonW is right, sorry for being obtuse.
The Omni has no idea what the IO module is.
All the interaction is via the UPB network.
So from the inputs, I have the Omni listen for the links the IO module transmits, and then take action based on them.
For the IO outputs, the Omni sends a UPB link to control it.
You can't send it a unit on command, the IO module only responds to links. So a "Kitchen Lights ON" type unit command, like "IO Module 1 ON", will have no effect.
You set up the links in Upstart and PC Access then send and receive the links.
Whether the Omni uses HLC or UPB, you can use spare links for anything you want.
I define all the links in my setup in the Omni, even those that don't directly control units, just to remind me.
I assign them unit numbers but they are in a "room" all their own.