What does it mean by "No System Effects are accounted for"?

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You want to read the Engineering reference, especially the sections Sizing Manager and Zone Design Loads and Air Flow Rates.

Bottom line is that the Design Load that is reported is based on a simulation with an ideal zonal system.

Quoting some sections from the Engineering reference:

The ZoneSizingCalc set to true signals the HVAC Manager to ignore the actual HVAC system and instead calculate the design zone loads and air flow rates using an ideal zonal system

[...]

SizeZoneEquipment assumes that each controlled zone is served by an ideal air conditioning unit. This unit supplies heating or cooling air at a fixed, user input temperature and humidity (specified in the Sizing:Zone objects). The units have infinite capacity: the flow rate can be any amount

Before the ideal zone load is calculated, the function checks whether the user wants to account for the heat gain or loss caused by the ventilation air from a Dedicated Outdoor Air System (DOAS). If the user has selected Account For Dedicated Outdoor Air = Yes the function performs an ideal DOAS calculatio

So it will not care whether you actually have a real system with a set maximum flow rate at the fan, or several zones to serve, etc.