As far as I know:
Zone:sizing: sizes zone equipment to fight zone thermal load. That thermal load does not include outside air if it is provided by a system. So if you have a system for mechanical ventilation and fancoils in the zone, the fancoils' coils autosized capacity is not enough to counteract thermal losses + outside aire provided by the system.I know you can autosize your system for ventilation requirement or for zone load + ventilation. That impacts on the capacity of the coils, fans, etc. But for zone equipment, sizing:zone mechanism does not add system load. It works as if air provided by the system is always neutral (preconditioned to zone conditions).As you say, the load is always calculated for the DesignDay but it does not include outdoor air load provided by a system (only infiltrations or natural ventilation).Regards.