Hi all, I would like to know what is meant by 100% fresh air and how this is achieved. Is this that the AHU is designed in such a way that the return air is not mixed back or is it something different. Please anyone clarify, thanks.

There is likely a heat exchanger that allows the exiting stale air to heat or cool in incoming fresh air to near the same temperature as practical. That allows 100% fresh (outside) air with a minimal extra operating costs.