In OpenStudio, after I define a return plenum, there appears to be a "Induced Air Outlet Port List" for the "OS:AirLoopHVAC:ReturnPlenum" and it has a drop down list of Port List 1 to Port List 12. I suppose it is for the secondary inlet nodes of fan powered mixing boxes(PIU), however I am not sure how to use this.
On the other hand, when I use either series or parallel PIU, I have no way to specify its secondary air inlet node in OpenStudio user interface.