Explanation of Polysurface Boolean Twisted Boxes keep type?

Does anyone know exactly what the keep type in this pufferfish component means?

It says “Determines how much of the trimmed twisted box set to keep 0 = Complete (keep if twisted box is completely to the keep side of the polysurface(Brep)) 1 = Halfway (keep if twisted box is atleast halfway to the keep side of the polysurface(Brep)) (fastest) 2 = Partial (keep if twisted box is atleast partially to the keep side of the polysurface(Brep))”

It does not seem to be completely accurate. For example, if the intersecting polysurface takes up a small enough part of the box using “partial” keep type, the box will not be kept. Is there a way of knowing how large this “part” needs to be to count as overlapping? What I am looking for is a component that tests for any overlap, am I missing something?