The unique identifier of the pointing device that generated this event.
Also known as tool ID, this is a unique number recorded in the chip inside every pointing device. The unique ID makes it possible to assign a specific pointing device to a specific tablet. You can also use it to “sign” documents or to restrict access to document layers to a specific pointing device. This method is valid for mouse events with subtype NSTabletProximityEventSubtype and for NSTabletProximity events.