Kawn fī al-ḥayyiz means a body's or substance's occupancy of a spatial location. The argument assumes that a body is in some locus, then compares its current occupation of that locus with what preceded it to classify the body as moving or at rest.
Ḥayyiz is a technical term within classical kalām, not an automatic equivalent of any modern scientific model of space or particles. The contrast concerns remaining in a place, moving to another, and the logical implications of those descriptions.
The assumption becomes disputable if an opponent allows entities that occupy no place. The reach of the premise must therefore be assessed rather than simply presumed.