You can use the priority attribute to determine whether the sense region is above or below its siblings. (The priority value can be any integer, positive or negative. Objects with higher priority are always on top of objects with lower priority.)  While this ordering has no visual effect, it does determine how objects are selected. For example, if two objects overlap at the point of a user selection, the higher of the two objects is selected. Placing a visible disabled sense region over other enabled objects prevents the user from selecting those objects.