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At the risk of sounding like a broken record lately, have you checked the EnergyPlus Input/Output Reference guide?

The section you're looking for is Controller:MechanicalVentilation:

Demand Controlled Ventilation

This field indicates whether the air loop is capable of doing demand controlled ventilation (DCV) to vary the amount of outdoor air based on actual number of occupants in spaces. Two choices: Yes and No. Default is No.

Basically say you have a space with a number of occupants $x$ and an occupancy schedule with a value of 0.5 between 0 and 12:00, and 0.8 between 12:00 and 24:00, meaning in your space you have always $0.5 x$ or $0.8 x$. And say you have specified a DesignSpecification:OutdoorAir Object that calls for $v$ CFM/people.

Based on the value of the Demand Controlled Ventilation field:

• If you select 'No': the amount of outdoor air is calculated using the nominal number of people X, so you always need $10 x . v$ CFM.
• If you select 'Yes': the amount of outdoor air will vary based on the number of occupant at each time step. Between 0 and 12:00, $5 x.v$ will be supplied and between 12:00 and 24:00 $8 x.v$ will be supplied