Chapter 4 sets how much outdoor air a space needs, driven mainly by occupancy. The ventilation-rate table (403.3.1.1 in recent editions) gives a per-person and a per-area rate by use, and the mechanical system has to deliver at least the sum. For spaces with swinging occupancy, demand-controlled ventilation is permitted; where the code allows it, natural ventilation through operable openings sized to the floor area can substitute for mechanical.
A common mistake is a mechanical schedule whose outdoor-air CFM doesn't reconcile with the occupancy and floor area, or a claim of natural ventilation without the required openable area actually shown.
Reviewers check that the provided outdoor air meets the tabulated rate for the use and occupant load — the ventilation calc and the occupancy have to agree.
This guide describes the model code for general understanding and is not a substitute for the adopted code and amendments enforced by your local authority having jurisdiction. Verify all figures against your jurisdiction's codes of record.