As the cfm of supply air is reduced in a VAV system, what happens to the cfm of outdoor air?

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In a VAV system, outdoor air is brought in as part of the total supply air through a mixing box, and the amount of outdoor air is controlled to meet ventilation requirements while also delivering the required cooling or heating to the zone. The outdoor-air flow is typically tied to the total supply air flow, either as a fixed fraction or as a function of the same control that sets the total air delivered to the space. When you reduce the overall supply air cfm to match a lower cooling or heating load, the amount of outdoor air that can be delivered with that air also drops, because there is less total air being sent to the space to carry outdoor air with it. So, as supply air is reduced, outdoor air cfm decreases as well, unless an independent control strategy is explicitly used to maintain a constant outdoor-air rate; in standard practice, it follows the reduction in total supply air.