The absolute z coordinate of pointing device on its tablet at full tablet resolution.
For the coordinate to be valid, this event must be generated by a tablet pointing device (otherwise the property is set to 0). The z coordinate does not represent pressure. It registers the depth coordinate returned by some tablet devices with wheels; if the device is something other than these, 0 is returned. This method is valid only for mouse events with a subtype of NSTabletPointEventSubtype and for events of type NSTabletPoint.