Economiser. The outside-air damper opened past its minimum on a cooling hour when the outside air is cooler, or carries less heat, than the return: free cooling. Economiser and outside air.
When the outside air is cooler than the return air, mixing more of it in cools the building for the price of the fan. An airside economiser is the outside-air damper opened past its minimum on those hours, under a changeover rule: a dry-bulb limit, or an enthalpy comparison in a humid climate where cool-looking air can carry more heat than the room's. ASHRAE 90.1 §6.5.1 requires one on most cooling systems above about 54 kW.
In a building simulation the damper is placed before any stage is judged, by solving the ventilation stage shut and open and interpolating to the supply the rooms need — mixing is exactly linear in damper position, so two solves are enough. Free cooling is a stage with no compressor in it, which is why a fan in AUTO keeps that stage after hours: a night purge is free, and dropping it once sent an empty telecom shelter's cooling to the compressor.
Open the builder — the examples in the Design tab's sources list include a machine of this kind, and the Simulate tab runs it against a year of weather.
DOAS — dedicated outside-air system · SZVAV — single-zone variable air volume · VAV terminal box · Waterside economiser · Demand-controlled ventilation · Face-and-bypass · Head-pressure control · Humidity ceiling and floor · Cycling and modulation · Coupling · Priority · Category · Nested system · Design point and TMY year