Three different air paths a BAS sequence must handle differently — one is meant to come back, one is surplus, and one must never return at all.
Look at a set of AHU control points and it's easy to lump "return air damper," "relief air damper," and "exhaust fan" together as three ways air leaves the occupied space. They aren't interchangeable, and a control sequence that treats them as the same category will either waste perfectly good air or risk letting contaminated air back into circulation. Each of these three air streams has a distinct job, a distinct destination, and a distinct reason it exists.