Trace 3D - modeling DOAS

Simple buttonolgy question for those of you who know Trace 3D. I have an office that is using VRF/DOAS. I have assigned each zone to the VRF in Trace, but how do I "direct" the right OA to the DOAS?

This is different than 700 because the DOAS seems like it's actually its own "system" not just a tab on the VRF system.