Two different ways to describe "how much fresh air" — and only one of them actually measures fresh air.
"This room gets 10 air changes an hour" sounds like a strong indoor air quality claim. It might be — or it might mean almost nothing about fresh air at all. Air changes per hour and outdoor air ventilation rate answer two genuinely different questions, and a space can score well on one while quietly failing the other. Mixing them up is one of the most common — and most consequential — misunderstandings in ventilation design, especially since one of them (ACH) rolled off the tongue constantly during pandemic-era air quality discussions while the code-enforced quantity (outdoor air rate) barely got mentioned.