Gerry,
Yes, I am using unicast.
This is what I did with the one model I created. Keep in mind that I still am waiting for China and only have 2 RGB strips here so I am having to move them around from port to port to see things blink.
I have the Pixcon, a 16 port controller with 510 channels per universe.
I created one model, a mega tree with 24 RGB strips. They are 2812b strips so there are 150 pixels per strip, or 450 channels.
16 strips is 7200 channels and all was good until I realized that 7200 is a channel number in the middle of universe 15. For me to use 7201 for the 17th strip which plugged into port #1 of the 2nd controller, I would have to tell xLights that universe 15 is both controllers, which you cannot do, so...
I plugged the 17th strip into port #1 of the 2nd controller and went back and changed the model to reflect that the 17th strip would start at the beginning of universe 16, which was channel 7651.
This means that channels 7201-7650 are not used. This "seems" to be OK with xLights---is this correct? Those unused channels just fall into a black hole and go nowhere but it doesn't matter.

Is this the correct way to define this?
Charles