The chip zone is simply an extension of the clear aperture, usually meant provide extra space for mounting the lens so that the mounting assembly doesn’t block light through the system.

You could also define this by adding that including distance in the mechanical semi-diameter, which is handled differently by OpticStudio. The surface area introduced by the chip zone has the curvature of the parent surface, extending it from the clear aperture to the full mechanical diameter. If you instead skip the chip zone and add to the mechanical semi-diameter manually, this extra distance will be flat (if no chip zone has been defined).