Because CO2 is a gas humans breath out, measuring a room’s CO2 levels is an effective way to know when a room is occupied and how many people are in that room.

CO2 levels are 400 ppm: then the room is assumed to be empty (no occupants) and a very minimum amount of ventilation needs to be provided.

CO2 levels reach 800-1200 ppm: then the room is assumed to be at an occupancy that requires additional ventilation beyond the minimum amount.

CO2 levels are greater than 1200 ppm: then the room is assumed to be occupied by more occupants than it was designed to accommodate. The ventilation rate should be at the maximum required amount for that room.